doc: modernize and fix code examples in https.md

* Replace `var` by `const`.
* Comment out ellipses.
* Update code example (provide relevant file path, add missing option).

PR-URL: https://github.com/nodejs/node/pull/12171


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>