Fixed the base class note in the Date Views docs.

    All of the generic views listed above have matching ``Base`` views that

    only differ in that they do not include the

    :class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`:

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

    (for the archive views) or

    :class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`

    (for the :class:`DateDetailView`):

    .. class:: BaseArchiveIndexView