GitLab

Minor update of README.md of language model example (#1063)

Summary:
With this white space, the command might fail.
```
fairseq-preprocess: error: unrecognized arguments:
zsh: command not found: --destdir
```
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1063

Differential Revision: D17072516

Pulled By: myleott

fbshipit-source-id: 68bb9d05b40b215b18aceac2bff3f5ec1ef2f537