docs: use const in readme example

fixes #3867
closes #3868

Author: mwanago

Readme.md

@@ -9,8 +9,8 @@
   [![Test Coverage][coveralls-image]][coveralls-url]
 
 ```js
-var express = require('express')
-var app = express()
+const express = require('express')
+const app = express()
 
 app.get('/', function (req, res) {
   res.send('Hello World')