Update macOS install docs to use the repo-root PHP server command

[Montekkundan]

The macOS install guide documents a PHP built-in server command that differs from the command used in CI.

repo

• macOS guide: php -S localhost:8000 -t . router.php
• CI: php -S localhost:8000 -t htdocs/ htdocs/router.php

Developers following the macOS guide may start the PHP server from the wrong directory / with the wrong document root, which can lead to routing issues and a confusing blank-page failure mode.

Expected outcome

Update the macOS docs to use the repo-root command