Inaccurate example Mapbox ID in README

[philgyford]

In the README, under "Mabox" it says:

You can get map_ID (i.e mapbox.satellite)...

But I think this format ("mapbox.satellite") is inaccurate since the change to the modern Static Tiles API.

The currently-available Mapbox styles are listed here and so the possible current strings to use as the id with leaflet-providers are:

• "mapbox/streets-v11"
• "mapbox/outdoors-v11"
• "mapbox/light-v10"
• "mapbox/dark-v10"
• "mapbox/satellite-v9"
• "mapbox/satellite-streets-v11"

So – they now use a slash rather than a period, and have version numbers appended.

[brunob]

Feel free to provide a PR for that :)