doc: fix missing backtick in fs.md

PR-URL: https://github.com/nodejs/node/pull/38260


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Nitzan Uziely <linkgoron@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>