doc: remove backtick escaping for manpage refs

Removing backticks will make the doctool emit links to the man pages.

PR-URL: https://github.com/nodejs/node/pull/9632


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>