fix(docs): clarify the default for tag_name (#544)

The logic here:
https://github.com/softprops/action-gh-release/blob/01570a1f39cb168c169c802c3bceb9e93fb10974/src/github.ts#L217

transforms `github.ref` in such a way that it matches `github.ref_name` instead. 

Per [docs](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/accessing-contextual-information-about-workflow-runs#github-context):

> github.ref_name	string	The short ref name of the branch or tag that triggered the workflow run. This value matches the branch or tag name shown on GitHub. For example, feature-branch-1.

Note, I could imagine a PR that changes the TypeScript code to use that variable instead, but I'm trying to be conservative here. Perhaps some users are on-prem GitHub Enterprise and that variable doesn't exist.

Author: alexeagle

README.md

@@ -186,7 +186,7 @@ The following are optional as `step.with` keys
 | `preserve_order`           | Boolean | Indicator of whether order of files should be preserved when uploading assets                                                                                                                                                                                                                                                                                                                                                                   |
 | `files`                    | String  | Newline-delimited globs of paths to assets to upload for release                                                                                                                                                                                                                                                                                                                                                                                |
 | `name`                     | String  | Name of the release. defaults to tag name                                                                                                                                                                                                                                                                                                                                                                                                       |
-| `tag_name`                 | String  | Name of a tag. defaults to `github.ref`                                                                                                                                                                                                                                                                                                                                                                                                         |
+| `tag_name`                 | String  | Name of a tag. defaults to `github.ref_name`                                                                                                                                                                                                                                                                                                                                                                                                         |
 | `fail_on_unmatched_files`  | Boolean | Indicator of whether to fail if any of the `files` globs match nothing                                                                                                                                                                                                                                                                                                                                                                          |
 | `repository`               | String  | Name of a target repository in `<owner>/<repo>` format. Defaults to GITHUB_REPOSITORY env variable                                                                                                                                                                                                                                                                                                                                              |
 | `target_commitish`         | String  | Commitish value that determines where the Git tag is created from. Can be any branch or commit SHA. Defaults to repository default branch.                                                                                                                                                                                                                                                                                                      |