doc: fix header level in fs.md

Indentation for #fs_fs_promises_api is level 2 but it should be level 3.

PR-URL: https://github.com/nodejs/node/pull/35771


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>