Modify code in "Grouped arithmetic" (#3904)

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

Author: Hossein-Madadi

doc/groupby.rst

@@ -157,7 +157,7 @@ This last line is roughly equivalent to the following::
 
     results = []
     for label, group in ds.groupby('letters'):
-        results.append(group - alt.sel(x=label))
+        results.append(group - alt.sel(letters=label))
     xr.concat(results, dim='x')
 
 Squeezing