build: update contributing guide to reflect branch name of code-of-conduct repository

Updates the contributing guide to reflect the branch name of `code-of-conduct` repository

Author: devversion

CONTRIBUTING.md

@@ -146,7 +146,7 @@ changes to be accepted, the CLA must be signed. It's a quick process, we promise
 
 
 [slack]: https://firebase-community.appspot.com/
-[coc]: https://github.com/angular/code-of-conduct/blob/master/CODE_OF_CONDUCT.md
+[coc]: https://github.com/angular/code-of-conduct/blob/main/CODE_OF_CONDUCT.md
 [commit-message-format]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#
 [corporate-cla]: https://code.google.com/legal/corporate-cla-v1.0.html
 [individual-cla]: http://code.google.com/legal/individual-cla-v1.0.html