Issue 1146: Minor fix to roberta pre-training readme (#1165)

Summary:
This is to make this instructions a little more generalizable, since in some systems, bash will parse the spaces within quotes

Addressing https://github.com/pytorch/fairseq/issues/1146
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1165

Differential Revision: D17547810

Pulled By: myleott

fbshipit-source-id: 5a026d42f678126b5ca8bc4477ba8f26ea549dcd

examples/roberta/README.pretraining.md

@@ -61,7 +61,7 @@ DATA_DIR=data-bin/wikitext-103
 fairseq-train --fp16 $DATA_DIR \
     --task masked_lm --criterion masked_lm \
     --arch roberta_base --sample-break-mode complete --tokens-per-sample $TOKENS_PER_SAMPLE \
-    --optimizer adam --adam-betas '(0.9, 0.98)' --adam-eps 1e-6 --clip-norm 0.0 \
+    --optimizer adam --adam-betas '(0.9,0.98)' --adam-eps 1e-6 --clip-norm 0.0 \
     --lr-scheduler polynomial_decay --lr $PEAK_LR --warmup-updates $WARMUP_UPDATES --total-num-update $TOTAL_UPDATES \
     --dropout 0.1 --attention-dropout 0.1 --weight-decay 0.01 \
     --max-sentences $MAX_SENTENCES --update-freq $UPDATE_FREQ \