Fix CORS handling code example

Example 1: what is cors

“CORS” stands for Cross-Origin Resource Sharing. 
It allows you to make requests from one website to another website 
in the browser, which is normally prohibited by another browser policy 
called the Same-Origin Policy (SOP).

Example 2: how to use cros

npm i cors
const cors = require('cors');
const express = require('express');
const app = express();
app.us(cors());