fix typo

ObservationSim/_util.py

@@ -13,7 +13,7 @@ def parse_args():
     '''
     parser = argparse.ArgumentParser()
     parser.add_argument('config_file', help='.yaml config file for simulation settings.')
-    parser.add_argument('-c', '--config_dir', help='Directory that houses the ,yaml config file.')
+    parser.add_argument('-c', '--config_dir', help='Directory that houses the .yaml config file.')
     parser.add_argument('-d', '--data_dir', help='Directory that houses the input data.')
     parser.add_argument('-w', '--work_dir', help='The path for output.')
     return parser.parse_args()