docs: app.del() with app.delete() in README example (#411)

The README pre-flight example used app.del() which has been deprecated since Express 4.x. Replaced it with the correct app.delete() method to align with current Express.js API and prevent confusion for new users.

Author: Vansh1811

README.md

@@ -150,7 +150,7 @@ var cors = require('cors')
 var app = express()
 
 app.options('/products/:id', cors()) // preflight for DELETE
-app.del('/products/:id', cors(), function (req, res, next) {
+app.delete('/products/:id', cors(), function (req, res, next) {
   res.json({msg: 'Hello'})
 })