Apptainer installation instructions are missing a step

[AntoineTheb]

Hello,

In the "requirements" section about the installation of apptainer/singularity, the instructions are
sudo apt-get update && sudo apt-get install -y apptainer

However, the ppa apptainer must be added first (https://apptainer.org/docs/admin/main/installation.html)

So the instructions should read

sudo add-apt-repository -y ppa:apptainer/ppa && sudo apt-get update && sudo apt-get install -y apptainer