Fix schema of `m.receipt` EDU (#1636)

It was missing a nesting level for the user ID -> user read receipt map.

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

changelogs/server_server/newsfragments/1636.clarification

@@ -0,0 +1 @@
+Fix schema of `m.receipt` EDU.

data/api/server-server/definitions/event-schemas/m.receipt.yaml

@@ -41,7 +41,11 @@ allOf:
             # on. At that point, m.read can become optional (maybe).
             "m.read":
               type: object
-              description: Read receipts for users in the room.
+              description: |-
+                Read receipts for users in the room. The string key is the user
+                ID the receipt belongs to.
+              additionalProperties:
+                type: object
                 title: User Read Receipt
                 properties:
                   event_ids: