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