README.md: Fix typo (#177)

# Fix a typo in the README

There was a typo in the `Adding new Extensions` chapter. It mentioned
the file `docs/_index.md` but I only found `docs/index.md`

## How to use

--

## Testing done

None

- [ ] Changelog entries added in the respective `changelog/` directory
(user-facing change, bug fix, security fix, update)
- [ ] Inspected CI output for image differences: `/boot` and `/usr`
size, packages, list files for any missing binaries, kernel modules,
config files, kernel modules, etc.

Author: tormath1

README.md

@@ -153,7 +153,7 @@ To add an extension:
    - `populate_sysext_root` populates the system extension root directory (available via `$sysextroot`).
       This function runs in a temporary directory; stuff can be downloaded / built in `./` and then moved to `$sysextroot`.
       The temporary directory will be removed when the extension build is complete (or when it errors out); no need to clean up manually.
-4. Add `docs/<my-extension-name>.md` with documentation and example configuration for the new extension, and reference the new file from the list of extensions in `docs/_index.md`.
+4. Add `docs/<my-extension-name>.md` with documentation and example configuration for the new extension, and reference the new file from the list of extensions in `docs/index.md`.
 5. Optionally add the new extension name to [release_build_versions.txt](https://github.com/flatcar/sysext-bakery/blob/main/release_build_versions.txt) to automatically build it when new upstream versions are released.
 6. File a PR.