Fixed typo and slightly improved error message when db is missing time zone definitions.

                if settings.USE_TZ:

                    if datetime is None:

                        raise ValueError("Database returned an invalid value "

                                         "in QuerySet.dates(). Are time zone "

                                         "definitions and pytz installed?")

                                         "in QuerySet.datetimes(). Are time zone "

                                         "definitions for your database and pytz installed?")

                    datetime = datetime.replace(tzinfo=None)

                    datetime = timezone.make_aware(datetime, self.query.tzinfo)

                yield datetime