Copy-edit CONTRIBUTING and update. (#18131)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18131
ghimport-source-id: 473dae70f6c236d317bec77d894310c0aa0376ec

Stack from [ghstack](https://github.com/ezyang/ghstack

):
* **#18131 Copy-edit CONTRIBUTING and update.**

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Differential Revision: D14505049

fbshipit-source-id: 02aeae33c0049889243c56dd0d761487dac2351e