`/_matrix/federation/v1/user/devices/{userId}`: Rename `self_signing_keys` to `self_signing_key` (#3312)

Apparently, in response to a /_matrix/federation/v1/user/devices/{userId} request, Synapse actually returns a key called "self_signing_key" instead of "self_signing_keys".

changelogs/server_server/newsfragments/3312.clarification

@@ -0,0 +1 @@
+Correct the `/_matrix/federation/v1/user/devices/{userId}` response which actually returns `"self_signing_key"` instead of `"self_signing_keys"`.

data/api/server-server/user_devices.yaml

@@ -96,7 +96,7 @@ paths:
                         "ed25519:base64+master+public+key": "base64+master+public+key",
                       }
                     }
-              self_signing_keys:
+              self_signing_key:
                 type: object
                 description: |-
                   The user\'s self-signing key.