stream: use arrow function for callback

In lib/internal/streams/async_iterator.js, use arrow function for
callback.

PR-URL: #24609
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/internal/streams/async_iterator.js

@@ -37,8 +37,8 @@ function onReadable(iter) {
 }
 
 function wrapForNext(lastPromise, iter) {
-  return function(resolve, reject) {
-    lastPromise.then(function() {
+  return (resolve, reject) => {
+    lastPromise.then(() => {
       iter[kHandlePromise](resolve, reject);
     }, reject);
   };