[skip ci] fixing argument parser

ml-agents/mlagents/trainers/cli_utils.py

     argparser.add_argument(
         "--plugins",
         default="",
-        nargs="*",
-        help="Absolute paths of plugins to be added to the trainers.",
+        type=str,
+        nargs=3,
+        help="Absolute paths of plugins to be loaded",
+        required=False
     )
     argparser.add_argument(
         "--env-args",

ml-agents/mlagents/trainers/settings.py

         :param args: collection of command-line parameters passed to mlagents-learn
         :return: RunOptions representing the passed in arguments, with trainer config, curriculum and sampler
           configs loaded from files.
+          configs loaded from files.
         """
         argparse_args = vars(args)
         config_path = StoreConfigFile.trainer_config_path