doc: fix typo in fs.md

PR-URL: https://github.com/nodejs/node/pull/40254


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Qingyu Deng <i@ayase-lab.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>