Fixed signature of ModelAdmin.get_paginator() in docs.

    documentation for more details. One difference is that the level may be

    passed as a string label in addition to integer/constant.

.. method:: ModelAdmin.get_paginator(queryset, per_page, orphans=0, allow_empty_first_page=True)

.. method:: ModelAdmin.get_paginator(request, queryset, per_page, orphans=0, allow_empty_first_page=True)

    Returns an instance of the paginator to use for this view. By default,

    instantiates an instance of :attr:`paginator`.