Fix a minor typo in docs/newforms.txt

The ``__unicode__`` method of the model will be called to generate

string representations of the objects for use in the field's choices;

to provide customized representations, subclass ``ModelChoiceField``

and override ``label_for_model``. This method will receive model

object, and should return a string suitable for representing it::

and override ``label_for_model``. This method will receive a model

object, and should return a string suitable for representing it. For

example::

    class MyModelChoiceField(ModelChoiceField):

        def label_from_instance(self, obj):