Grammatically update the readme docs (#48328)
Summary:
Small grammatical update to the readme docs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48328
Reviewed By: linbinyu
Differential Revision: D25132876
Pulled By: mrshenli
fbshipit-source-id: f1214b3098bec6713ef53f226f8d0d33946a5ec1
