fix JobsArgument --help output

src/flake8/main/options.py

@@ -82,6 +82,10 @@ class JobsArgument:
                 "{!r} must be 'auto' or an integer.".format(arg),
             )
 
+    def __str__(self):
+        """Format our JobsArgument class."""
+        return "auto" if self.is_auto else str(self.n_jobs)
+
 
 def register_default_options(option_manager):
     """Register the default options on our OptionManager.

tests/unit/test_option_manager.py

@@ -373,3 +373,9 @@ def test_parse_invalid_jobs_argument(optmanager, capsys):
         "'foo' must be 'auto' or an integer.\n"
     )
     assert expected in output
+
+
+def test_jobs_argument_str():
+    """Test that JobsArgument has a correct __str__."""
+    assert str(JobsArgument("auto")) == "auto"
+    assert str(JobsArgument("123")) == "123"