doc: fix usage of folder and directory terms in fs.md

This commit fixes the interchangeably usage of "folder" and "directory"
terms in fs.md

Fixes: https://github.com/nodejs/node/issues/32902

PR-URL: https://github.com/nodejs/node/pull/32919


Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>