docs: Update contributing (eu-digital-green-certificates#19)

Author: daniel-eder

CONTRIBUTING.md

@@ -48,7 +48,7 @@ The following rule governs documentation contributions:
 
 ## Pull Request Checklist
 
-* Branch from the master branch and, if needed, rebase to the current master branch before submitting your pull request. If it doesn't merge cleanly with master you may be asked to rebase your changes.
+* Branch from the main branch and, if needed, rebase to the current main branch before submitting your pull request. If it doesn't merge cleanly with main you may be asked to rebase your changes.
 
 * Commits should be as small as possible while ensuring that each commit is correct independently (i.e., each commit should compile and pass tests).