doc: fix position of `fs.readSync()`

PR-URL: https://github.com/nodejs/node/pull/6399


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>