One of the first comments in the guide says: “Donc à partir d’un Volumio 1.55 d’origine”
So it’s based on a very outdated version of Volumio.
The step you refer to has this comment: “5/ Ajout de la commande dans l’interface Web de Volumio”
It seems to point to the web interface, which has gotten a massive overhaul in Volumio 2. Not only are these files no longer in this location, the entire method of calling the player controls has changed.
Check the plugin sections here, there might already be a power-down button to achieve what you want . Or try to update the guide and upload the plugin yourself
The good thing is that with Volumio 2, external control has become much easier. Simply find out which command is needed for proper shutdown, and send that command when the button is pressed.