Fix a broken link in CONTRIBUTING.md (#44701)
Summary:
as the title says :)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44701
Reviewed By: ngimel
Differential Revision: D23724919
Pulled By: mrshenli
fbshipit-source-id: 5ca5ea974ee6a94ed132dbe7892a9b4b9c3dd9be
