doc: use reserved domains for examples in url.md

Changes non-reserved domains to reserved ones in url.md.

PR-URL: https://github.com/nodejs/node/pull/23359
Refs: https://www.iana.org/domains/reserved


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>