📝 Change master ref to main in contrib doc

Author: webknjaz

.github/CONTRIBUTING.rst

@@ -3,7 +3,7 @@ Contributing to |project|
 =========================
 
 Make sure you read the `README
-<https://github.com/cherrypy/cheroot/blob/master/README.rst>`_.
+<https://github.com/cherrypy/cheroot/blob/main/README.rst>`_.
 Also **ensure you set up pre-commit utility correctly** and
 tests pass in GitHub Actions CI/CD workflows.