Mark the appservice ping response duration_ms field as required

As intended in MSC2659.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/api/client-server/appservice_ping.yaml

@@ -82,6 +82,8 @@ paths:
                     The duration in milliseconds that the
                     [`/_matrix/app/v1/ping`](#post_matrixappv1ping)
                     request took from the homeserver's point of view.
+            required:
+              - duration_ms
           examples:
             application/json: {"duration_ms": 123}
         400: