Minor changes to Sgr coordinate frame example - #5637

This is definitely an improvement and pretty trivial, so I'm going to merge it now so that it can be in the 1.3 docs.

I think what @mhvk meant is something like the plotting quantities example, right? I think in this case that's not really useful because angular plots already have the "degree" symbol there - and for matplotlib angular projections it's important that it be provided as radians anyway, otherwise the projection stuff goes super wonky.

So I'll merge this, but keep #5555 open on the theory that @mhvk should have the final say there on whether or not this satisfies that issue.

I did indeed mean also using the matplotlib units methods, but that was less important than what was done here: have obvious unit conversions without using quantity. So, fine to just leave it at this!