Fix grammar typo in readme.md (#50000)
Summary:
missing `
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50000
Reviewed By: ezyang
Differential Revision: D25759608
Pulled By: mrshenli
fbshipit-source-id: 4dbe06b8978ae5b2b9b66cde163dab4bd8ee2257
