Link to xarray-spatial is wrong

[felixcremer]

The link to the xarray-spatial docs is wrong and leads to some shady looking site.

See also xarray-contrib/xarray-spatial#811 for details.

I think this should rather link to https://xarray-spatial.readthedocs.io/en/stable/user_guide/zonal.html

[martinfleis]

Heh, seems that someone at xarray-spatial forgot to extend the domain... Thanks for the heads up!