Fixed typo in docs/ref/models/custom-lookups.txt.

.. method:: get_transform(lookup_name)

    Django uses ``get_transform(lookup_name)`` to fetch lookups. The

    Django uses ``get_transform(lookup_name)`` to fetch transforms. The

    implementation of ``get_transform()`` looks for a subclass which is registered

    for the current class with the correct ``transform_name``.