url: fix remaining calculation

Fix remaining calculation in the PercentDecode function to match the
definition in URL standard: https://url.spec.whatwg.org/#remaining

PR-URL: https://github.com/nodejs/node/pull/15637


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>