Update an URL (#1542)

Author: srittau

docs/source/writing_stubs.rst

@@ -91,7 +91,7 @@ things like detecting missing annotations to more complex things like ensuring
 Liskov substitutability or detecting problematic overloads.
 
 It may be instructive to examine `typeshed <https://github.com/python/typeshed/>`__'s
-`setup for testing stubs <https://github.com/python/typeshed/blob/master/tests/README.md>`__.
+`setup for testing stubs <https://github.com/python/typeshed/blob/main/tests/README.md>`__.
 
 ..
    TODO: consider adding examples and configurations for specific type checkers