doc: fix fs.realpath man pg links

There is no realpath(2), only in sections 1 & 3
causes broken link on nodejs.org/api/ docs

PR-URL: https://github.com/nodejs/node/pull/6451


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>