docs: fix apt mirror restore commands to match by path

[tangzz-radxa]

Summary

Fixes the "Restore Defaults" instructions in the Radxa OS apt repository mirror docs (_apt-repo.mdx, zh-CN + en).

Problem

The restore block used two sequential domain-only sed replacements per mirror, e.g.:

sudo sed -i 's/mirrors.cqu.edu.cn/deb.debian.org/g' /etc/apt/sources.list.d/*
sudo sed -i 's/mirrors.cqu.edu.cn/deb.ubuntu.com/g' /etc/apt/sources.list.d/*

The first command rewrites every occurrence of the mirror host — including Ubuntu source paths — to deb.debian.org, so the second command never sees mirrors.cqu.edu.cn again. On an Ubuntu system, restoring defaults yields broken entries like deb.debian.org/ubuntu instead of deb.ubuntu.com/ubuntu.

This was originally flagged by Copilot review on #1781 (comments on both the en and zh files), but #1781 was merged with the issue still present in main.

Fix

Match by path instead of host only, so each distribution is restored to its own default host:

sudo sed -i 's|mirrors.cqu.edu.cn/debian|deb.debian.org/debian|g' /etc/apt/sources.list.d/*
sudo sed -i 's|mirrors.cqu.edu.cn/ubuntu|deb.ubuntu.com/ubuntu|g' /etc/apt/sources.list.d/*

Applied to all affected mirror blocks (ustc, tuna, cqu) in both docs/common/radxa-os/_apt-repo.mdx and the en counterpart.

Test / verification

• Bilingual guard: en + zh both updated, no missing counterparts.
• The path-based pattern is consistent with the enable commands (host-only swap preserves the /debian vs /ubuntu path).