[watchmedo] (minor) check top_command instead of command in watchmedo…

….main() (#890)

src/watchdog/watchmedo.py

@@ -635,7 +635,7 @@ def handler_termination_signal(_signum, _frame):
 def main():
     """Entry-point function."""
     args = cli.parse_args()
-    if args.command is None:
+    if args.top_command is None:
         cli.print_help()
     else:
         args.func(args)