Positive cell detection workflow table shows wrong metric

[bethac07]

Thanks as always for the great work!

I think there may be a wrong figure used in the positive cell detection tutorial - the text seems to be referring to an OD measurement which may be negative, but the value graphed is area (and is always therefore nonnegative). I spot checked a couple of versions and it's always an area graph, even in 0.2 when the graph is different than it is in stable and latest. To avoid user confusion, the graph should probably be of OD, and possibly be negative in some places.