doc: conform to rules for eslint-plugin-markdown

PR-URL: https://github.com/nodejs/node/pull/12563
Refs: https://github.com/nodejs/node/pull/12557#issuecomment-296015032


Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>