Change SUBDOMAINS spec string so that it works on Stamen's SSL Fastly

I also found that the instructions given at http://maps.stamen.com/#usage-ssl for multiple subdomains over SSL have the wrong URL specification. It says to use https://stamen-tiles-{S}.a.ssl.fastly.net but https://stamen-tiles-{S}a.ssl.fastly.net is the correct spec.