Fix POST _matrix/federation/v1/user/keys/claim response schema

The syntax was not compliant with the Swagger spec.
It also lacked one level of nesting.

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

data/api/server-server/user_keys.yaml

@@ -76,12 +76,15 @@ paths:
 
                   See the [Client-Server Key Algorithms](/client-server-api/#key-algorithms) section for more information on
                   the Key Object format.
+                # User
                 additionalProperties:
                   type: object
+                  # Device
                   additionalProperties:
-                    type:
-                      - string
-                      - type: object
+                    type: object
+                    # Key
+                    additionalProperties:
+                      type: object
                       title: KeyObject
                       properties:
                         key: