doc: update abort signal in fs promise api example

PR-URL: https://github.com/nodejs/node/pull/38669


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>