Docs: fix verbiage for enforced 2FA (#205)

We're not requiring them to be a superuser [to use 2FA], we're requiring them to use 2FA if they're a superuser.

Author: dragon-dxw

docs/advanced.rst

@@ -10,7 +10,7 @@ or being in a particular group). This is implemented by subclassing the
 ``require_2fa`` method on it. This middleware needs to be added to your
 ``MIDDLEWARE_CLASSES`` setting.
 
-For example, to require a user to be a superuser:
+For example, to require superusers to use 2FA:
 
 .. code-block:: python