doc: fixed variable names in queueMicrotask example

fixed variable name in queueMicrotask where url variable was used
instead of key

PR-URL: https://github.com/nodejs/node/pull/39634


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>