[Fix]修改ubuntu18文档错误

docs/setup_by_ubuntu18.rst

@@ -264,6 +264,7 @@ Ubuntu 18.04 安装文档
     $ apt-get -y install curl gnupg2 ca-certificates lsb-release
     $ add-apt-repository "deb http://nginx.org/packages/ubuntu/ $(lsb_release -cs) nginx"
     $ curl -fsSL http://nginx.org/keys/nginx_signing.key | sudo apt-key add -
+    $ apt-get update
     $ apt-get -y install nginx
 
 **6.2 准备配置文件 /etc/nginx/conf.d/jumpserver.conf**