Did some much-needed rewriting/editing in the formsets section of...

Did some much-needed rewriting/editing in the formsets section of docs/topics/forms/modelsforms.txt. 'It self' is not two words.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9616 bcc190cf-cafb-0310-a4f2-bffc1f526a37