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>

doc/api/fs.md

@@ -2209,7 +2209,7 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [Readable Stream]: stream.html#stream_class_stream_readable
 [Writable Stream]: stream.html#stream_class_stream_writable
 [inode]: https://en.wikipedia.org/wiki/Inode
-[FS Constants]: #fs_fs_constants
+[FS Constants]: #fs_fs_constants_1
 [`inotify`]: http://man7.org/linux/man-pages/man7/inotify.7.html
 [`kqueue`]: https://www.freebsd.org/cgi/man.cgi?kqueue
 [`FSEvents`]: https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/FSEvents_ProgGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40005289-CH1-SW1