[1.3.X] Fixed #16145 - typo in manager docs; thanks leereilly.

Set ``use_for_related_fields`` when you define the class

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``use_for_related_fields`` attribute must be set on the manager *class*,

object not on an *instance* of the class. The earlier example shows the

correct way to set it, whereas the following will not work::

The ``use_for_related_fields`` attribute must be set on the manager *class*, not

on an *instance* of the class. The earlier example shows the correct way to set

it, whereas the following will not work::

    # BAD: Incorrect code

    class MyManager(models.Manager):