Tiny change to docstring in many_to_one_null model unit test

git-svn-id: http://code.djangoproject.com/svn/django/trunk@558 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/testapp/models/many_to_one_null.py

@@ -1,7 +1,8 @@
 """
 16. Many-to-one relationships that can be null
 
-To define a many-to-one relationship, use ``ForeignKey()`` with ``null=True`` .
+To define a many-to-one relationship that can have a null foreign key, use
+``ForeignKey()`` with ``null=True`` .
 """
 
 from django.core import meta