[Contracts] Fix branch name in README.md links

Author: chalasr

README.md

@@ -6,4 +6,4 @@ A set of abstractions extracted out of the Symfony components.
 Can be used to build on semantics that the Symfony components proved useful - and
 that already have battle tested implementations.
 
-See https://github.com/symfony/contracts/blob/master/README.md for more information.
+See https://github.com/symfony/contracts/blob/main/README.md for more information.