[1.9.x] Fixed #26477 -- Fixed typo in docs/ref/contrib/contenttypes.txt

This enables filtering, ordering, and other query operations on ``Bookmark``

from ``TaggedItem``::

    >>> # Get all tags belonging to books containing `django` in the url

    >>> # Get all tags belonging to bookmarks containing `django` in the url

    >>> TaggedItem.objects.filter(bookmarks__url__contains='django')

    [<TaggedItem: django>, <TaggedItem: python>]