[1.7.x] Replaced an instance of PIL with Pillow in the docs.

Done in 4965a77407 on master.

docs/ref/models/fields.txt

@@ -841,9 +841,9 @@ optional arguments:
     Name of a model field which will be auto-populated with the width of the
     image each time the model instance is saved.
 
-Requires the `Python Imaging Library`_.
+Requires the `Pillow`_ library.
 
-.. _Python Imaging Library: http://www.pythonware.com/products/pil/
+.. _Pillow: http://python-imaging.github.io/Pillow/
 
 By default, :class:`ImageField` instances are created as ``varchar(100)``
 columns in your database. As with other fields, you can change the maximum