doc: fix unsafe writable stream code example

Update writable stream code example using async iterator to use safer
`finished()` method instead of a `finish` event to avoid uncaught
exceptions

Fixes: https://github.com/nodejs/node/issues/29397

PR-URL: https://github.com/nodejs/node/pull/29425


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>