Miscellaneous small doc fixes (#15373)
Summary:
This PR makes some small changes for better consistency in our README and
CONTRIBUTING docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15373
Differential Revision: D13512753
Pulled By: driazati
fbshipit-source-id: 44398ad1894eef521d5f5acb1d06acaad67728cf
