Fixed #15134 -- Placed formset documentation before modelform documentation in...

Fixed #15134 -- Placed formset documentation before modelform documentation in the forms topic index to improve the logical flow of information being presented. Thanks to Aryeh Leib Taurog for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15447 bcc190cf-cafb-0310-a4f2-bffc1f526a37