pull_request_template.md: fix link to the Contributor Guide (#1178)

* pull_request_template.md: fix link to the `Contributor Guide`

Used a relative link

* Update .github/pull_request_template.md

Author: jbampton

.github/pull_request_template.md

@@ -5,7 +5,7 @@
 <!-- If this is your first PR in the Lucene.NET repo, please run through the checklist
 below to ensure a smooth review and merge process for your PR. -->
 
-- [ ] You've read the [Contributor Guide](https://github.com/apache/lucenenet/blob/main/CONTRIBUTING.md) and [Code of Conduct](https://www.apache.org/foundation/policies/conduct.html).
+- [ ] You've read the [Contributor Guide](https://github.com/apache/lucenenet/blob/master/CONTRIBUTING.md) and [Code of Conduct](https://www.apache.org/foundation/policies/conduct.html).
 - [ ] You've included unit or integration tests for your change, where applicable.
 - [ ] You've included inline docs for your change, where applicable.
 - [ ] There's an open issue for the PR that you are making. If you'd like to propose a change, please [open an issue](https://github.com/apache/lucenenet/issues/new/choose) to discuss the change or find an existing issue.