tools: build API TOC using raw headers

Markdown interprets the syntax for optional arguments as a form
of a link, so instead of trying to build up the contents using
the node value, use grab the raw original markup instead.

Fixes regression noted in
https://github.com/nodejs/node/pull/21490#issuecomment-406785023

PR-URL: https://github.com/nodejs/node/pull/21922


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>