doc: fix arg definition in fs

Currently doc building doesn't support ES-style default params in
function definitions which causes an error.

PR-URL: https://github.com/nodejs/node/pull/18678


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Gus Caplan <me@gus.host>