Fixed typo in docs.

        ``object_list`` is the list of objects ``extra_context`` is a

        dictionary of context data that will be added to any context data

        provided by the

        :class:`~django.db.views.generic.list.MultiplObjectMixin`.

        :class:`~django.views.generic.list.MultipleObjectMixin`.

    .. method:: get_dated_queryset(**lookup)