Fix typo in method name
Summary:
It was consistently wrong so it worked before, but not having a typo is
better.
Test Plan: Compiles, grepped that it's not invokable/public
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D2266