Fix typo in config group name

Summary: Quickly fix it before anyone relies on this typo.

Test Plan: None. Yes, really untested.

Reviewers: Plasma, graesslin, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kwin, plasma-devel, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D11047