Modify code in "Grouped arithmetic"

[Hossein-Madadi]

results.append(group - alt.sel(x=label)) must be changed to:
results.append(group - alt.sel(letters=label))

[max-sixty]

Thanks @hoseinmadadi , good spot.

Want to add to whatsnew and give yourself credit?

[Hossein-Madadi]

@max-sixty, yes thanks, It will be my honor.

[max-sixty]

Great, ping when added and tests pass, thanks!

[max-sixty]

@hoseinmadadi do you want to add a line to the whatsnew file?

[shoyer]

Let's just merge. I would always encourage contributors to give themselves credit, but in this case that's almost more of a burden than the fix itself :)

@hoseinmadadi thanks for the fix here! if you want to add a credit to yourself in whatsnew in a later PR, feel free!