docs(contributing.rst): fix parameter value in example test code

The value must be False according to the case test.

CONTRIBUTING.rst

@@ -687,7 +687,7 @@ useful when testing components that depend on each other:
 
     def test_downgrade_not_superuser_skips_email(factories, mocker):
         mocked_notify = mocker.patch('funkwhale_api.myapp.notifications.notify')
-        user = factories['users.User'](is_superuser=True)
+        user = factories['users.User'](is_superuser=False)
         users.downgrade_user(user)
 
         # here, we ensure no email was sent