Fixed #13336 -- Corrected an incorrect example of cache header usage. Thanks...

Fixed #13336 -- Corrected an incorrect example of cache header usage. Thanks to dnjuls for the report, and claudep for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13129 bcc190cf-cafb-0310-a4f2-bffc1f526a37