[kcmkwin/kwinoptions] Fix default for MaximizeButtonRightClickCommand

Summary:
KWin assumes that the default value for MaximizeButtonRightClickCommand
is Maximize (horizontal only).

Reviewers: #kwin, broulik

Reviewed By: broulik

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29673