[1.8.x] Fixed typo docs/ref/models/relations.txt

        the ``blog`` :class:`~django.db.models.ForeignKey` doesn't have

        ``null=True``, this is invalid.

        .. versionchanged 1.7::

        .. versionchanged:: 1.7

        For :class:`~django.db.models.ForeignKey` objects, this method accepts

        a ``bulk`` argument to control how to perform the operation.