Fixed #6982; added an import statement to example in docs/modelforms.txt for the ModelForm class

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7411 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/modelforms.txt

@@ -121,6 +121,7 @@ A full example
 Consider this set of models::
 
     from django.db import models
+    from django.newforms import ModelForm
 
     TITLE_CHOICES = (
         ('MR', 'Mr.'),