Removed extra new line in error message

Author: kmvanbrunt

cmd2/argparse_completer.py

@@ -231,7 +231,7 @@ def update_mutex_groups(arg_action: argparse.Action) -> None:
                         if arg_action == completer_action:
                             return
 
-                        error = ("Error: argument {}: not allowed with argument {}\n".
+                        error = ("Error: argument {}: not allowed with argument {}".
                                  format(argparse._get_action_name(arg_action),
                                         argparse._get_action_name(completer_action)))
                         raise CompletionError(error)