fix contributing link

Author: christian-bromann

readme.md

@@ -10,7 +10,7 @@ We intend for this icon pack to be used with [Ionic](http://ionicframework.com/)
 ## Contributing
 
 Thanks for your interest in contributing! Read up on our guidelines for
-[contributing](https://github.com/ionic-team/ionicons/blob/master/.github/CONTRIBUTING.md)
+[contributing](https://github.com/ionic-team/ionicons/blob/main/.github/CONTRIBUTING.md)
 and then look through our issues with a [help wanted](https://github.com/ionic-team/ionicons/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22)
 label.