[kcmkwin/decorations] Use correct header text
Summary: The header text is supposed to be the KCM's name, not a duplicate of the tooltip text.
Test Plan:
Reviewers: VDG, GB_2
Reviewed By: VDG, GB_2
Subscribers: kwin
Tags: KWin
Maniphest Tasks: T10273
Differential Revision: https://phabricator.kde.org/D19018