Fix Debian guidelines in portal error message

Closes #31.

portal_error.txt

@@ -2,16 +2,9 @@ Your Flatpak version does not support a feature needed for this Chromium Flatpak
 to run. This generally happens when your distribution has unprivileged user
 namespaces disabled.
 
-========== FOR DEBIAN USERS ==========
+========== FOR DEBIAN FAMILY USERS ==========
 
-If the distro you are currently using is Debian Buster (10) or Sid, then this
-has already been fixed in the latest system updates. If you are using Buster,
-install the 'bubblewrap' and 'flatpak' from the buster-backports repository;
-guidance on setting up backports is available at the following URL:
-
-https://backports.debian.org/Instructions/
-
-For older Debian versions or other Debian-based distros, you can run the
+If the distro you are currently using is Debian or a derivative, you can run the
 following commands as root (you can enter a root shell via 'sudo -i'):
 
 dpkg-statoverride --update --add root root 0755 /usr/bin/bwrap