`IntervalBox` not defined error in Kelvin-Helmholtz example

The [Kelvin-Helmholtz example](https://tomchor.github.io/Oceanostics.jl/stable/generated/kelvin_helmholtz/) currently has an error message right after the figure and axes definition:  I couldn't reproduce this error locally and I also have no idea where it comes from. It seems to not be a showstopper (the example still runs and animates successfully) but it would be nice to get rid of it.