Fix for SUNRepresentations type parameter change

[leburgel]

Small fix in the symmetric tensor tutorial for SUNRepresentations v0.4, where SUNIrrep{3} has become an abstract type. It anyway made no sense to mix SU3Irrep and SUNIrrep{3} in the same line, looks needlessly confusing.

If we want, I can also bump the SUNRepresentations compat entry to v0.4 in the docs project, so we can use SUNRepresentations.casimir instead of having to define our own casimir method. It's quite pedagogical the way it is so I didn't do this right away, but if this would be better I'll change it.

[lkdvos]

I would actually be in favor of using SUNRepresentations.casimir as that helps visibility of that method, and I think the pedagogical part is more about using it than defining it?

Btw, could you then also update the [compat] in the docs Project.toml?

[github-actions]

After the build completes, the updated documentation will be available here

[codecov]

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