update README.md.

更新说明

README.md

@@ -12,14 +12,17 @@
 
 > 最好是使用云主机进行安装，虚拟主机有很多局限性
 
-> git clone -b 4.0 https://gitee.com/sentcms/sentcms.git sentcms
 
-> cd sentcms
+```
+git clone -b 4.0 https://gitee.com/sentcms/sentcms.git sentcms
+cd sentcms
+```
 
 把根目录下的.example.env文件复制一份并命名为.env
 
-> composer install
+```
-
+composer install
+```
 
 访问网址：http://网址/install