update async example for cookie validation middleware

en/guide/writing-middleware.md

@@ -146,16 +146,16 @@ When you make a request to the root of the app, the app now displays the timesta
 
 Finally, we'll create a middleware function that validates incoming cookies and sends a 400 response if cookies are invalid.
 
-Here's a simple function that validates the presence of certain cookies.
+Here's a simple function that validates cookies with an external async service.
 
 ```js
 function cookieValidator (cookies) {
   return new Promise(function (resolve, reject) {
-    if (!cookies.testCookie) {
-      reject(new Error('Invalid cookies'))
-    }
-
-    resolve()
+    externallyValidateCookie(cookies.testCookie)
+      .then(resolve)
+      .catch(function () {
+        reject(new Error('Invalid cookies'))
+      })
   })
 }
 ```
@@ -169,10 +169,9 @@ var cookieValidator = require('./cookieValidator')
 
 var app = express()
 
-function validateCookies (req, res, next) {
-  return cookieValidator(req.cookies).then(function () {
-    next()
-  })
+async function validateCookies (req, res, next) {
+  await cookieValidator(req.cookies)
+  next()
 }
 
 app.use(cookieParser)