add nginx.conf modification guide

ubuntu_18_04_LTS_安裝_R45_手冊

@@ -24,6 +24,7 @@ $ sudo systemctl status nginx # 查看 nginx 是否啟動
 $ sudo systemctl restart nginx # 安全重啟 nginx，若已啟動則可跳過此步驟
 $ cd /etc/nginx/
 $ sudo mkdir orbit_sites/
+$ vim nginx.conf # add `include /etc/nginx/orbit_sites/*;` to Virtual Host Configs block
 $ sudo vim orbit_sites/xxx_ooo # 建立網站 nginx 設定檔，xxx 為學校縮寫，ooo 為系所、單位縮寫。內容參考附註 nginx 設定檔
 ```
 ```nginx