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