doc: fix doc example for cctest

cctest guide example wasn't working because of a few typos.

PR-URL: https://github.com/nodejs/node/pull/17355


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>