doc: fixes formatting in process

Fixes several formatting errors in the process doc, including missing
link references, misplaced underscores, and a missing backtick.

Fixes: https://github.com/nodejs/node/issues/9223
PR-URL: https://github.com/nodejs/node/pull/9235


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>