fix readme (#2939)
fix readme (#2939)

Summary:
minor fix

Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/2939

Reviewed By: michaelauli

Differential Revision: D33685330

Pulled By: alexeib

fbshipit-source-id: 4d6c6edb1fab9d0d56a6e03c0a2b43a864f1d07a

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq