Update README.md (#679)
Update README.md (#679)

Summary:
Add missing backslash.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/679

Differential Revision: D15122270

Pulled By: myleott

fbshipit-source-id: fbdfde648051294eaa9f7a4e0c4cfbc57491a718

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq