doc: fix fs constants link

https://nodejs.org/api/fs.html#fs_fs_constants links to itself rather
than to https://nodejs.org/api/fs.html#fs_fs_constants_1

PR-URL: https://github.com/nodejs/node/pull/9508


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>