Add another entry to categories

Otherwise desktop-file-validate barfs thusly:

mpDris2.desktop.orig: error: (will be fatal in the future): value item
"Audio" in key "Categories" in group "Desktop Entry" requires another
category to be present among the following categories: AudioVideo

mpDris2.desktop.orig: hint: value item "Music" in key "Categories" in
group "Desktop Entry" can be extended with another category among the
following categories: AudioVideo, or Education