Missing Comment in desktop file

Summary:
Added a comment in the desktop file in Kturtle.

BUG: 351968

Reviewers: KTurtle, cfeck, yurchor

Reviewed By: yurchor

Subscribers: yurchor, cfeck, kde-edu, KTurtle

Tags: KTurtle, KDE Edu

Differential Revision: https://phabricator.kde.org/D17376