Update README.md

example code in readme failed in browser client by message: "Uncaught DOMException: Failed to execute 'send' on 'WebSocket': Still in CONNECTING state" so wait for connection state is opened

README.md

@@ -86,5 +86,5 @@ class MyChat implements MessageComponentInterface {
     // Then some JavaScript in the browser:
     var conn = new WebSocket('ws://localhost:8080/echo');
     conn.onmessage = function(e) { console.log(e.data); };
-    conn.send('Hello Me!');
+    conn.onopen = function(e) { conn.send('Hello Me!'); };
 ```