Fix micro readme example (#4827)

I found mistake on readme

readme.md

@@ -848,7 +848,7 @@ const micro = require('micro')
 
 const dev = process.env.NODE_ENV !== 'production'
 const app = next({ dev })
-const handle = app.getRequestHandler()
+const handleNextRequests = app.getRequestHandler()
 
 app.prepare().then(() => {
   const server = micro((req, res) => {