Fix Broken Link in CONTRIBUTING.md (#41066)
Summary:
Spotted a broken link, and while I was at it, fixed a few little language and formatting nits.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41066
Reviewed By: mruberry
Differential Revision: D22415371
Pulled By: dongreenberg
fbshipit-source-id: 7d11c13235b28a01886063c11a4c5ccb333c0c02
