Update Swift Chat metadata example

Reflects the fact that we now use JSONValue (see ably-chat-swift commit
c360d7c).

content/chat/rooms/reactions.textile

@@ -150,5 +150,5 @@ const MyComponent = () => {
 try await room.reactions.send(params: .init(type: "like"))
 
 try await room.reactions.send(params: .init(type: "heart",
-                              metadata: ["effect": .string("fireworks")]))
+                              metadata: ["effect": "fireworks"]))
 ```