Update contribution_guide.rst (#64142)
Summary:
Grammatical update.
Fixes #{issue number}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64142
Reviewed By: mruberry
Differential Revision: D30639394
Pulled By: ezyang
fbshipit-source-id: cf1a4dfbd8e34b0772f1b09f5d820278e8ef8574
