vector_layers for raster tilesets

[pnorman]

The current text of 3.0.0 has a couple of problems for raster layers

Note: When describinng a set of raster tiles or other tile format that does not have a "layers" concept (i.e. "format": "jpeg"), the vector_layers key is not required.

There's a few issues that should be changed for 3.0.1

1. Typo in "describinng"
2. use of "i.e." In a case where there is an example, it should be e.g., not i.e., as the latter says jpeg is the only raster tile format
3. Referencing format. Previous versions of the spec had a format key, but that is no longer defined in the spec, so it's unclear to users what "format": "jpeg" means
4. Making it optional. The text earlier in the section says vector_layers is required, but this changes that. Instead of sorting that out, I recommend making it optional. For vector tiles with a layer concept, it's not always necessary to know what all the layers are in order to make use of the tiles.

[pnorman]

"implemenntation" is another typo, which makes me think someone had a sticky "n" key.