[docs] Use path.join instead of concatenating paths (#3014)

Author: Stanley288

examples/chat/index.js

@@ -1,6 +1,7 @@
 // Setup basic express server
 var express = require('express');
 var app = express();
+var path = require('path');
 var server = require('http').createServer(app);
 var io = require('../..')(server);
 var port = process.env.PORT || 3000;
@@ -10,7 +11,7 @@ server.listen(port, function () {
 });
 
 // Routing
-app.use(express.static(__dirname + '/public'));
+app.use(express.static(path.join(__dirname, 'public')));
 
 // Chatroom