Remove warning in upcoming cellpose/stardist segmentation jupyter notebook

[elevans]

An in progress PR (https://github.com/imagej/pyimagej/pull/283) that brings conversion support for index images to ROIs also has a jupyter notebook detailing how to use it. This notebook has a warning stating that you need to use the main branch of PyImageJ to use the new features. Once we release the next version of PyImageJ (with these new features) we should remove this warning.