stream: use arrow function for callback

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

PR-URL: https://github.com/nodejs/node/pull/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>