Wording fix - objects contain keys, not list elements directly

specification/server_server_api.rst

@@ -673,7 +673,8 @@ invoked using the room ID and the event ID of the new member event.
 The resident homeserver then accepts this event into the room's event graph,
 and responds to the joining server with the full set of state for the newly-
 joined room. This is returned as a two-element list, whose first element is the
-integer 200, and whose second element contains the following keys:
+integer 200, and whose second element is an object which contains the
+following keys:
 
 ============== ===== ============
  Key            Type  Description