Made a minor tweak to a doctoring in `django/forms.forms.py` that was missed...

Made a minor tweak to a doctoring in `django/forms.forms.py` that was missed in r17452. Thanks to Claude Paroz for the tip.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17453 bcc190cf-cafb-0310-a4f2-bffc1f526a37