doc: clarify special schemes

In url.md, describe "special" protocol schemes as "special" rather than
"considered to be special".

PR-URL: https://github.com/nodejs/node/pull/28091


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>