修改Centos7更换阿里源文档不足

Docs/Centos7更换阿里源.md

@@ -12,8 +12,8 @@
 2. 下载阿里云Yum源
 
    ```bash
-   curl -O http://mirrors.aliyun.com/repo/Centos-7.repo
-   curl -O http://mirrors.aliyun.com/repo/epel-7.repo
+   curl -so /etc/yum.repos.d/Centos-7.repo http://mirrors.aliyun.com/repo/Centos-7.repo
+   curl -so /etc/yum.repos.d/epel-7.repo http://mirrors.aliyun.com/repo/epel-7.repo
    ```
 
 3. *删除缓存数据*
@@ -27,5 +27,3 @@
    ```bash
    yum makecache
    ```
-
-