Use better KDE GTK Config KCM desktop file comment

Summary:
Use a better KDE GTK Config KCM desktop file comment that is correct and consistent with others.

Test Plan: Open the KDE GTK Config KCM.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19009