Update docs

- Ref: hwdsl2#1097

Author: hwdsl2

README-zh.md

@@ -88,7 +88,7 @@ wget https://git.io/vpnquickstart -O vpn.sh && sudo sh vpn.sh
 
 ## 安装说明
 
-首先，更新你的系统：运行 `apt-get update && apt-get dist-upgrade` (Ubuntu/Debian) 或者 `yum update` 并重启。这一步是可选的，但推荐。
+首先，更新你的系统：运行 `sudo apt-get update && sudo apt-get dist-upgrade` (Ubuntu/Debian) 或者 `sudo yum update` 并重启。这一步是可选的，但推荐。
 
 要安装 VPN，请从以下选项中选择一个：
 

README.md

@@ -88,7 +88,7 @@ A pre-built [Docker image](https://github.com/hwdsl2/docker-ipsec-vpn-server) is
 
 ## Installation
 
-First, update your system with `sudo apt-get update && sudo apt-get dist-upgrade` (Ubuntu/Debian) or `yum update` and reboot. This is optional, but recommended.
+First, update your system with `sudo apt-get update && sudo apt-get dist-upgrade` (Ubuntu/Debian) or `sudo yum update` and reboot. This is optional, but recommended.
 
 To install the VPN, please choose one of the following options: