update the url for specification documents for the MBTiles tileset format

[d3netxer]

This url: https://docs.mapbox.com/help/how-mapbox-works/#mbtiles

just points to the general MapBox help for me. I'm not sure if you want to point to the mbtiles page in the glossary (https://docs.mapbox.com/help/glossary/mbtiles/) or somewhere else.