doc: fix stream async iterator sample

The for await loop into writable loop could cause an unhandled exception
in the case where we are waiting for data from the async iterable and
this no `'error'` handler is registered on the writable.

Fixes: https://github.com/nodejs/node/issues/31222

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


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