Fix obvious PlasmaCore -> Kirigami issues
This fixes the links from PlasmaCore.Units by pointing them to Kirigami.Units as per plasma/plasma-workspace#74 (closed).
This MR is just for fixing the current pipeline failures, so I only changed superficial things.
It is not a rewrite or main fix of the tutorial, so don't review it as such. That would come in a future MR.