Fix readme

Author: danschultzer

README.md

@@ -120,7 +120,7 @@ test "WebSocketClient" do
   {:ok, socket} = TestServer.websocket_init("/ws")
 
   :ok = TestServer.websocket_handle(socket)
-  :ok = TestServer.websocket_handle(socket, to: fn {:text, "ping"}, state -> {:reply, "pong", state} end)
+  :ok = TestServer.websocket_handle(socket, to: fn {:text, "ping"}, state -> {:reply, {:text, "pong"}, state} end)
   :ok = TestServer.websocket_handle(socket, match: fn {:text, message}, _state -> message == "hi")
 
   {:ok, client} = WebSocketClient.start_link(TestServer.url("/ws"))