Fixed #11810 -- Fixed typo and errors that prevented `modifiable` from working...

Fixed #11810 -- Fixed typo and errors that prevented `modifiable` from working in the geographic admin.  Thanks to Rob Coup for the bug report.  Refs #12504.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12995 bcc190cf-cafb-0310-a4f2-bffc1f526a37