fix spelling in code comment
fix spelling in code comment

mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile