README: update link to metro extracts
README: update link to metro extracts

As noted on http://metro.teczno.com/ :
```
This collection is no longer maintained. 

The fine folks at Mapzen have built a newer and more up-to-date version of Metro Extracts. Get data from mapzen.com/metro-extracts and suggest changes at mapzen/metroextractor-cities.
```
saydulk/osm-bright,jeffaudi/osm-hybrid,raumobil/osm-bright,silph-io/osm-silph,mapbox/osm-bright,pinkflozd/osm-bright,brelevenix/osm-bright-tregor,Sweetymeow/osm-bright