[1.6.x] Fixed typo in docs/topics/testing/tools.txt.

    used in ``TestCase``, no actual commit or rollback will be performed by the

    database. This can cause your tests to pass or fail unexpectedly. Always

    use ``TransactionTestCase`` when testing transactional behavior or any code

    that can't normally be excuted in autocommit mode

    that can't normally be executed in autocommit mode

    (:meth:`~django.db.models.query.QuerySet.select_for_update()` is an

    example).