Default branch is now main

To update, I ran it
$ sed -i '' -e 's/master/main/g' CONTRIBUTING.md

Author: neko314

CONTRIBUTING.md

@@ -14,7 +14,7 @@ Pull requests are the best way to propose changes to the codebase.
 
 We actively welcome your pull requests:
 
-1. Fork the repo and create your branch from `master`.
+1. Fork the repo and create your branch from `main`.
 2. If you've added code that should be tested, add tests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.