Fixed #21490 -- Fixed custom admin URL reverse example.

if you specifically wanted the admin view from the admin instance named

``custom``, you would need to call::

    >>> change_url = urlresolvers.reverse('custom:polls_choice_change', args=(c.id,))

    >>> change_url = urlresolvers.reverse('custom:polls_choice_change',

    ...                                   args=(c.id,), current_app='custom')

For more details, see the documentation on :ref:`reversing namespaced URLs

<topics-http-reversing-url-namespaces>`.