Add magic tag to execute install code

Add the magic tag before the installation snipped, such that doxec can execute
it.

README.md

@@ -21,6 +21,7 @@ additional packages `numpy`, `scipy` and `matplotlib` are useful for data
 analysis and data presentation. To install all the packages on Ubuntu, you
 can run the following command line.
 
+<!-- console -->
 ```bash
 $ sudo apt-get update
 $ sudo apt-get install -y python3 python3-pip