fixed the learn.py with a better way

python/learn.py

     _USAGE = '''
     Usage:
       learn (<env>) [options]
-
-    Execute with --help to get more information
+      learn --help
-      --help                     Show this message.
       --curriculum=<file>        Curriculum json file for environment [default: None].
       --keep-checkpoints=<n>     How many model checkpoints to keep [default: 5].
       --lesson=<n>               Start learning from this lesson [default: 0].