Update src/um_fields_subset.py

Co-authored-by: Davide Marchegiani <davide.marchegiani@gmail.com>

src/um_fields_subset.py

@@ -46,7 +46,7 @@ def parse_args():
                         help="Only include prognostic variables (sections 0, 33 and 34). Cannot be used together with --include or --exclude.")
     meg.add_argument('--include', dest='include_list',  type=convert_to_list, metavar="STASH_CODES",
                         help="Comma-separated list of STASH codes to include in the output file. Any STASH code present in the input file, but not contained in this STASH code list, will not be present in the output file. Cannot be used together with --prognostic or --exclude.")
-    meg.add_argument('--exclude', dest='exclude_list',  type=convert_to_list, metavar="STASH_CODE",
+    meg.add_argument('--exclude', dest='exclude_list',  type=convert_to_list, metavar="STASH_CODES",
                         help="Comma-separated list of STASH codes to exclude from the output file. All STASH codes present in the input file, but not contained in this STASH code list, will be present in the output file. Cannot be used together with --prognostic or --include.")
     parser.add_argument('--validate', action='store_true',
                         help='Validate the output fields file using mule validation.')