doc: fix WHATWG URL url.protocol example

Protocol of `https://example.org` is `https:` not `http:`.

PR-URL: https://github.com/nodejs/node/pull/11647


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>