doc: rename iterator to iterable in examples

PR-URL: https://github.com/nodejs/node/pull/31252
Fixes: https://github.com/nodejs/node/issues/31222


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>