feat: modify R repository command

The new command automatically fetches the codename of the distribution to avoid student adding a wrong ppa and having to remove it.

11_install_system_programs.Rmd

@@ -78,10 +78,10 @@ You can check the list of repositories that `apt` checks in the file `/etc/apt/s
 You can add the official cran repository to your repositories list:
 
 ```sh
-sudo add-apt-repository 'deb https://cloud.r-project.org/bin/linux/ubuntu <release_name>-cran40/'
+sudo add-apt-repository "deb https://cloud.r-project.org/bin/linux/ubuntu $(lsb_release -cs)-cran40/"
 ```
 
-You can use the command `lsb_release -sc` to get your **release name**.
+The command `lsb_release -sc` automatically fetch your distribution **release name**.
 
 Then you must add the public key of this repository: