Fixed #6047 -- Minor correction to the documentation regarding regex field...

Fixed #6047 -- Minor correction to the documentation regarding regex field lookups. Thanks to Richard D. Worth for the suggestion, and Steve Holden for the text. 

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11541 bcc190cf-cafb-0310-a4f2-bffc1f526a37