Fixed a typo in docs/model-api.txt

cannot use a string to reference a model defined in another application or

imported from elsewhere.

**New in Django development version:** to refer to models defined in another

application, you must instead explicitially specify the application label. That

is, if the ``Manufacturer`` model above is defined in another application called

``production``, you'd need to use::

**New in Django development version:** To refer to models defined in another

application, you must instead explicitly specify the application label. For

example, if the ``Manufacturer`` model above is defined in another application

called ``production``, you'd need to use::

    class Car(models.Model):

        manufacturer = models.ForeignKey('production.Manufacturer')