lib: fix readFile flag option typo

PR-URL: https://github.com/nodejs/node/pull/35292


Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>