Corrected admin.E023 message in docs.
@@ -623,7 +623,8 @@ with the admin site:
* **admin.E022**: The value of ``radio_fields`` refers to ``<field name>``,
which is not an attribute of ``<model>``.
* **admin.E023**: The value of ``radio_fields`` refers to ``<field name>``,
- which is not a ``ForeignKey``, and does not have a ``choices`` definition.
+ which is not instance of ``ForeignKey``, and does not have a ``choices``
+ definition.
* **admin.E024**: The value of ``radio_fields[<field name>]`` must be either
``admin.HORIZONTAL`` or ``admin.VERTICAL``.
* **admin.E025**: The value of ``view_on_site`` must be either a callable or a
