[1.8.x] Fixed typo in docs/topics/db/aggregation.txt.

    >>> Author.objects.aggregate(average_rating=Avg('book__rating'))

(The resulting dictionary will have a key called ``'average__rating'``. If no

(The resulting dictionary will have a key called ``'average_rating'``. If no

such alias were specified, it would be the rather long ``'book__rating__avg'``.)

Aggregations and other QuerySet clauses