Fix contribution_guide docs (#18237)
Summary:
Fixes Typo and a Link in the `docs/source/community/contribution_guide.rst`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18237
Differential Revision: D14566907
Pulled By: ezyang
fbshipit-source-id: 3a75797ab6b27d28dd5566d9b189d80395024eaf
