[1.2.X] Fixed id attribute generation in the admin docs page. Patch from simeon.

Fixed #3695. Thanks.

Backport of r13728 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13747 bcc190cf-cafb-0310-a4f2-bffc1f526a37