fixed cmd arg for shuffle dataset masked lm task

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/584

Reviewed By: myleott

Differential Revision: D15360774

Pulled By: myleott

fbshipit-source-id: b18efbb6ff5a8832c61b689f3d87c958cbd908e9

fairseq/tasks/masked_lm.py

@@ -41,7 +41,7 @@ class MaskedLMTask(FairseqTask):
                             help='max number of total tokens over all segments'
                                  ' per sample for BERT dataset')
         parser.add_argument('--break-mode', default="doc", type=str, help='mode for breaking sentence')
-        parser.add_argument('--shuffle-dataset', default=False)
+        parser.add_argument('--shuffle-dataset', action='store_true', default=False)
 
     def __init__(self, args, dictionary):
         super().__init__(args)