a little change

get_url.js

@@ -1,7 +1,7 @@
 const xhr = new XMLHttpRequest()
 xhr.open('GET', 'http://www.wechall.net/challenge/training/programming1/index.php?action=request')
 xhr.send(null)
-xhr.onreadystatechange = function(){
+xhr.onreadystatechange = () => {
     const DONE = 4
     const OK = 200
     if (xhr.readyState === DONE) {