doc: remove confusing signature in fs.md

PR-URL: https://github.com/nodejs/node/pull/18310
Fixes: https://github.com/nodejs/node/issues/18305
Refs: https://github.com/nodejs/node/pull/13424


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>