update polka example (#5370)

[polka](https://github.com/lukeed/polka/releases/tag/v0.5.0) changed their listen api to be more like express like

examples/custom-server-polka/package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "next": "latest",
-    "polka": "0.2.3",
+    "polka": "0.5.1",
     "react": "16.2.0",
     "react-dom": "16.2.0"
   }

examples/custom-server-polka/server.js

@@ -15,7 +15,8 @@ app.prepare().then(() => {
 
   server.get('*', (req, res) => handle(req, res))
 
-  server
-    .listen(port)
-    .then(() => console.log(`> Ready on http://localhost:${port}`))
+  server.listen(PORT, err => {
+    if (err) throw err;
+    console.log(`> Ready on http://localhost:${PORT}`)
+  })
 })