New string to be translated in gnome-session

[Didier Roche]

Dear translators,

Once gnome-session 3.26.1-0ubuntu4 hits the archive, you will see a new 
string to be translated: "Ubuntu on Xorg", this one is the session name 
that is available on gdm (login screen). People installing gnome-session 
can see a similar "GNOME on Xorg" string which is translated upstream, 
hence our decision.

The fact that it wasn't translatable was a typo which got fixed once 
https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/1715822 got 
raised.

Thanks, and sorry for this last minute fix :)
Cheers,
Didier