Fix typos in PGFTS docs.

    [<Entry: Cheese on Toast recipes>, <Entry: Pizza Recipes>]

This creates a ``to_tsvector`` in the database from the ``body_text`` field

and a ``plainto_tsquery`` from the search term ``'Potato'``, both using the

and a ``plainto_tsquery`` from the search term ``'Cheese'``, both using the

default database search configuration. The results are obtained by matching the

query and the vector.

if it were an annotated ``SearchVector``::

    >>> Entry.objects.update(search_vector=SearchVector('body_text'))

    >>> Entry.objects.filter(search_vector='potato')

    >>> Entry.objects.filter(search_vector='cheese')

    [<Entry: Cheese on Toast recipes>, <Entry: Pizza recipes>]

.. _PostgreSQL documentation: http://www.postgresql.org/docs/current/static/textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS