更换中国科学技术大学的软件源

deb http://mirrors.ustc.edu.cn/debian stable main contrib non-free
# deb-src http://mirrors.ustc.edu.cn/debian stable main contrib non-free
deb http://mirrors.ustc.edu.cn/debian stable-updates main contrib non-free
# deb-src http://mirrors.ustc.edu.cn/debian stable-updates main contrib non-free
deb http://mirrors.ustc.edu.cn/debian stable-proposed-updates main contrib non-free
# deb-src http://mirrors.ustc.edu.cn/debian stable-proposed-updates main contrib non-free
deb http://mirrors.ustc.edu.cn/debian-security/ stable-security main non-free contrib
# deb-src http://mirrors.ustc.edu.cn/debian-security/ stable-security main non-free contrib