doc: fix typographic error in process doc

An apostrophe was being used where a backtick was called for, resulting
in improper rendering.

PR-URL: https://github.com/nodejs/node/pull/7431


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