minor [Contracts] Fix branch name in README.md links

Author: chalasr

CHANGELOG.md

@@ -2,4 +2,4 @@ CHANGELOG
 =========
 
 The changelog is maintained for all Symfony contracts at the following URL:
-https://github.com/symfony/contracts/blob/master/CHANGELOG.md
+https://github.com/symfony/contracts/blob/main/CHANGELOG.md