Fix api-ref with Sphinx 1.5

Sphinx 1.5 fails with a warning that a code-block cannot be highlighted.
It's indeed not json, use 'none' to fix the build.

Change-Id: I02bfe805c419fcc49ac43b66f4f7b1a0d1d54755

Author: ajaeger

api-ref/source/v2/images-parameters.yaml

@@ -162,7 +162,7 @@ sort:
     Use the comma (``,``) character to separate multiple values. For
     example:
 
-    .. code-block:: json
+    .. code-block:: none
 
        GET /v2/images?sort=name:asc,status:desc
   in: query