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

examples/language_model/README.md

@@ -51,7 +51,7 @@ fairseq-preprocess \
     --only-source \
     --trainpref $TEXT/wiki.train.tokens \
     --validpref $TEXT/wiki.valid.tokens \
-    --testpref $TEXT/wiki.test.tokens \ 
+    --testpref $TEXT/wiki.test.tokens \
     --destdir data-bin/wikitext-103 \
     --workers 20
 ```