Arrange a few titles (#1126)

* Arrange a few titles

Before this change, PublicRoomsChunk in the spec text could be found in
two places (actually three but the third one is identical to the first
one), defining two (_mostly_ identical but) different schemas.
Ctrl-F'ing through the spec may confuse you with the "wrong" definition.
This commit gives distinct schemas distinct names; aside from
PublicRoomChunk, the same story happens with the derivative of
stripped_state.yaml used in space_hierarchy.yaml (it's not exactly
StrippedStateEvent either).

As for the removal of `PublicRoomsChunks` (plural) - this title is
unnecessary because the tooling would place rather self-explanatory
`[PublicRoomsChunk]` without it, whereas the plural title ends up with
no definition in the spec text.

* Add changelog

data/api/client-server/space_hierarchy.yaml

@@ -127,6 +127,7 @@ paths:
                   allOf:
                     - $ref: "definitions/public_rooms_chunk.yaml"
                     - type: object
+                      title: ChildRoomsChunk
                       properties:
                         room_type:
                           type: string
@@ -143,6 +144,7 @@ paths:
                             allOf:
                               - $ref: "../../event-schemas/schema/core-event-schema/stripped_state.yaml"
                               - type: object
+                                title: StrippedChildStateEvent
                                 properties:
                                   origin_server_ts:
                                     type: number