Fix schema of m.mentions object (#1635)

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

data/api/client-server/definitions/m.mentions.yaml

@@ -18,17 +18,13 @@ description: |-
   Describes whether the event mentions other users or the room. This is contained
   within the event's `content` alongside other fields for the relevant event type.
 example: {
-    "body": "Hello Alice!",
-    "msgtype": "m.text",
-    "format": "org.matrix.custom.html",
-    "formatted_body": "Hello <a href='https://matrix.to/#/@alice:example.org'>Alice</a>!",
-    "m.mentions": {
-        "user_ids": ["@alice:example.org"]
-    }
+    "user_ids": ["@alice:example.org"]
 }
 properties:
   user_ids:
-    type: string[]
+    type: array
+    items:
+      type: string
     description: A list of Matrix IDs of mentioned users.
   room:
     type: boolean