Fixed #10039 -- More typos in aggregation docs. Seriously, people, now you're...

Fixed #10039 -- More typos in aggregation docs. Seriously, people, now you're just making me look bad :-) Thanks to ElliottM, and to Erich Holscher for a separate report that I've piggybacked on this checkin.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9754 bcc190cf-cafb-0310-a4f2-bffc1f526a37