Fixed #15138 -- Clarified a slightly ambiguous example in the custom template...

Fixed #15138 -- Clarified a slightly ambiguous example in the custom template tag docs. Thanks to elbarto for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15448 bcc190cf-cafb-0310-a4f2-bffc1f526a37