Removed a stray reference to mysql_old in the docs.

-------

`MySQLdb`_ is the Python interface to MySQL. Version 1.2.1p2 or later is

required for full MySQL support in Django. Earlier versions will not work with

the ``mysql`` backend.

If you are trying to use an older version of MySQL and the ``mysql_old``

backend, then 1.2.0 *might* work for you.

required for full MySQL support in Django.

.. note::

    If you see ``ImportError: cannot import name ImmutableSet`` when trying to