fix: update link to universal blue contributing guide in PR template (ublue-os#1083)

Previous link was a 404

Author: aaron-trout

.github/pull_request_template.md

@@ -4,7 +4,7 @@ Here are some tips for you:
 
 ## Thank you for contributing to the Universal Blue project!
 
-Please [read the Contributor's Guide](https://universal-blue.org/CONTRIBUTING/) before submitting a pull request.
+Please [read the Contributor's Guide](https://universal-blue.org/contributing.html) before submitting a pull request.
 
 In this project we follow [Semantic PRs][1] and [Conventional Commits][2] to describe features and fixes we made. It would be nice if you did too as we use this to generate changelogs with the right sections: