Fix default colour used by Swap monitor

Summary:
It has a typo, meaning it gives an error (in a completely unrelated
place...because QML) and doesn't actually render anything.

Test Plan: No more error

Reviewers: Plasma, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11559