Fix the response for `GET /_matrix/app/v1/thirdparty/protocol/{protocol}` (#3675)

This should return a single protocol, not all of them.

Fixes #2286.

changelogs/application_service/newsfragments/3675.clarification

@@ -0,0 +1 @@
+Correct the documentation for the response value for `GET /_matrix/app/v1/thirdparty/protocol/{protocol}`.

data/api/application-service/protocols.yaml

@@ -48,7 +48,7 @@ paths:
         200:
           description: The protocol was found and metadata returned.
           schema:
-            $ref: definitions/protocol_metadata.yaml
+            $ref: definitions/protocol.yaml
         401:
           description: |-
             The homeserver has not supplied credentials to the application service.