Fix name of natural_color composite in quickstart

[djhoese]

As discussed on slack, the quick start document uses old composite names.

• Fully documented

[mraspaud]

LGTM

[djhoese]

@mraspaud As you've noticed in other PRs, the tests are failing. One failure I see is our use of func that is passed to Xarray's coarsen. The func keyword doesn't seem to have ever existed. It is called coord_func. However, I don't understand how this wasn't found before. It's been this way for many versions. Or maybe it is just supposed to be passed as a positional argument: https://github.com/pydata/xarray/pull/3500/files#diff-3fbfbfa64dfac6984aaa9d3925bf57beL1867

[mraspaud]

Interesting...