[CentOS-mirror] Update domain for centos.ustc.edu.cn => mirrors.ustc.edu.cn

Hi,

We have changed our mirror domain from "centos.ustc.edu.cn" to 
"mirrors.ustc.edu.cn", and CentOS mirror is under 
"http(s)://mirrors.ustc.edu.cn/centos/"

Now it supports HTTP, HTTPS and Rsync 
(rsync://rsync.mirrors.ustc.edu.cn/repo/centos/). The DNS records of 
centos.ustc.edu.cn will be removed soon.

Thank you.

-- 
Linux User Group
University of Science and Technology of China
Homepage: http://lug.ustc.edu.cn/
E-Mail: lug at ustc.edu.cn
Keyu Tao (陶柯宇) <taoky at ustclug.org>