Ros tutorial Error about rosdep init or rosdep update

Because the old version about ubuntu 16.04 for ros, the traditional method of init and update in no effictive. So we shoud download and modify something to complete rosdep update.

1. git clone https://github.com/ros/rosdistro

2. Modify 20-default.list of web URL to Local file. We can study from: https://blog.youkuaiyun.com/ainvgkridl/article/details/118755464

3. Modify Python code URL to Local file. We can study part of method 3 from: https://blog.youkuaiyun.com/qq_30267617/article/details/115028689?utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.control&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7Edefault-1.control