[1.2.X] Fixed #13397 - Include third level headings in the TOC. thanks cyang...

[1.2.X] Fixed #13397 - Include third level headings in the TOC. thanks cyang for the suggestion, rleland for the patch.

Backport of r15073 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15074 bcc190cf-cafb-0310-a4f2-bffc1f526a37