Small fixes to the Contributing.txt (#63385)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63385
Correcting a mistake for the pytorch uninstall, and
adding an extra note for Darwin.
Test Plan: Imported from OSS
Reviewed By: janeyx99, heitorschueroff
Differential Revision: D30530234
fbshipit-source-id: e0f88a1725eeadabfb4b28c1da11e369ee878ab4
