docs: update upstream branch to main

Author: zimeg

.github/CONTRIBUTING.md

@@ -47,4 +47,4 @@ If the contribution doesn't meet the above criteria, you may fail our automated
 3. :herb: Create a new branch and check it out.
 4. :crystal_ball: Make your changes and commit them locally. Magic happens here!
 5. :arrow_heading_up: Push your new branch to your fork. (e.g. `git push username fix-issue-16`).
-6. :inbox_tray: Open a Pull Request on github.com from your new branch on your fork to `master` in this repository.
+6. :inbox_tray: Open a Pull Request on github.com from your new branch on your fork to `main` in this repository.