[doc] fix broken link to TileJSON in doc

[OttyLab]

Summary

A link to TileJSON was broken and fixed it.

• Old: https://www.mapbox.com/developers/tilejson/
• New: https://docs.mapbox.com/api/maps/#retrieve-tilejson-metadata

Additional information

I just fixed for v3.3.1 (latest). Do I need to do the same to older versions?

[mrmanc]

Thanks for your speedy action @OttyLab!