Fix the type of children in the /hierarchy S-S API. (#3660)

* Fix the type of children in the /hierarchy S-S API.

* Newsfragment

changelogs/server_server/newsfragments/3660.new

@@ -0,0 +1 @@
+Add the Space Hierarchy API (`GET /_matrix/federation/v1/hierarchy/{roomId}`) as per [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946).

data/api/server-server/space_hierarchy.yaml

@@ -160,9 +160,11 @@ paths:
                               required: [origin_server_ts]
                     required: [room_type, allowed_room_ids, children_state]
               children:
+                type: array
                 description: |-
                   A summary of the space's children. Rooms which the requesting server cannot peek/join will
                   be excluded.
+                items:
                   allOf:
                     - $ref: "../client-server/definitions/public_rooms_chunk.yaml"
                     - type: object