Docs: The main branch is "main" now

Author: matthias-mayr

CONTRIBUTING.md

@@ -24,7 +24,7 @@ Please try to include as much information as you can. Details like these are inc
 Contributions via pull requests are much appreciated.
 Before sending us a pull request, please ensure that:
 
-1. You are working against the latest source on the *master* branch.
+1. You are working against the latest source on the *main* branch.
 2. You check existing open, and recently merged, pull requests to make sure someone else hasn't addressed the problem already.
 3. You open an issue to discuss any significant work - we would hate for your time to be wasted.