change `coords` to `coordinates` for valid geojson

Bonus: [if the files use `.geojson`](https://github.com/kjell/tgn-geojson/tree/dot-geojson), [github renders them with leaflet](https://github.com/kjell/tgn-geojson/blob/dot-geojson/data/701/408/0/7014080.geojson).