change python orders.md

How to change python version default in Ubuntu 20.04

1. Check the installed versions of Python:

$ ls /usr/bin/python*

2. Then, create the alternatives:

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 1
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 2

3. Then, choose the version you want:

$ sudo update-alternatives --config python