Update rm_source.md

Let the source link more standardized

current_docs/quick_start/rm_source.md

@@ -16,7 +16,7 @@ wget -O - https://rmsource.gdutelc.com/key/public.key | sudo apt-key add -
 ## 添加源
 将软件源添加到apt的sources.list文件。
 ```sh
-echo "deb https://rmsource.gdutelc.com/ focal main" | sudo tee -a /etc/apt/sources.list
+echo "deb https://rmsource.gdutelc.com/ubuntu/ focal main" | sudo tee -a /etc/apt/sources.list
 ```
 更新软件源。
 ```sh
@@ -25,4 +25,4 @@ sudo apt update
 完成以上操作后，即可在apt list中查找到可安装的最新版本的rm-controls。
 ```sh
 sudo apt list ros-noetic-rm*
-```
+```