`IntervalBox` not defined error in Kelvin-Helmholtz example

[tomchor]

The Kelvin-Helmholtz example 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.

[glwagner]

Does it depend on the Makie version?

[tomchor]

It's possible. But we don't commit a Manifest.toml for the docs, so I don't know if we can control for that.