add debian non-free packages to code example

Example: debian 10 sources

$ cat /etc/apt/sources.list

deb http://httpredir.debian.org/debian buster main non-free contrib
deb-src http://httpredir.debian.org/debian buster main non-free contrib

deb http://security.debian.org/debian-security buster/updates main contrib non-free
deb-src http://security.debian.org/debian-security buster/updates main contrib non-free