Fixed typo in docs/ref/checks.txt.

docs/ref/checks.txt

@@ -208,7 +208,7 @@ with the admin site:
 * **admin.E023**: The value of ``radio_fields`` refers to ``<field name>``,
   which is not a ``ForeignKey``, and does not have a ``choices`` definition.
 * **admin.E024**: The value of ``radio_fields[<field name>]`` must be either
-  ``admin.HORIZONTAL`` nor ``admin.VERTICAL``.
+  ``admin.HORIZONTAL`` or ``admin.VERTICAL``.
 * **admin.E025**: The value of ``view_on_site`` must be either a callable or a
   boolean value.
 * **admin.E026**: The value of ``prepopulated_fields`` must be a dictionary.