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

Author: devversion

CONTRIBUTING.md

@@ -233,7 +233,7 @@ changes to be accepted, the CLA must be signed. It's a quick process, we promise
   [print, sign and one of scan+email, fax or mail the form][corporate-cla].
 
 
-[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/preview
 [corporate-cla]: http://code.google.com/legal/corporate-cla-v1.0.html
 [github]: https://github.com/angular/flex-layout