doc: specify default encoding in writable.write

Refs: https://github.com/nodejs/node/issues/33715

PR-URL: https://github.com/nodejs/node/pull/33765


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