doc: fix invalid path doc comments

The format of certain code comments in the `path` documentation results
in the code blocks being invalid. I also find it confusing at least as
formatted on the website. This change is intended to improve those
comments.

PR-URL: https://github.com/nodejs/node/pull/5670


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>