Fix minor typo in contributing.md (#67855)
Summary:
Fixes #{issue number}
No issue number, minor change
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67855
Reviewed By: malfet
Differential Revision: D32186689
Pulled By: driazati
fbshipit-source-id: 7cda19f66ff1312296d8310922bb0d221df81e46
