[1.8.x] Fixed #24186 -- Fixed a typo in the admin docs.

Thanks to Keryn Knight for the report.

Backport of dccf41cff0 from master

docs/ref/contrib/admin/index.txt

@@ -663,7 +663,7 @@ subclass::
 
       .. versionadded:: 1.7
 
-      To indicate descending order with ``admin_model_field`` you can use a
+      To indicate descending order with ``admin_order_field`` you can use a
       hyphen prefix on the field name. Using the above example, this would
       look like::