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.
{F6614949}

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