[doc] incorrect description of vertexCount field in loaded 3D Tile content

[andreasplesch]

The documentation at

loaders.gl/docs/specifications/category-3d-tiles.md

Line 102 in 5f4973a

| `vertexCount` | `Number` | Transforms tile geometry positions to fixed frame coordinates |

has a copy and paste bug for the vertexCount field's description of its content. The description is copied over from the modelMatrix field which of course does not apply.
An appropriate description may read instead
Number of vertices in the loaded tile
I file this here under Discussions since the issue template did not cover documentation bugs.

[ibgreen]

@andreasplesch Thanks for reporting. We can look into adding something for documentation bugs, but feel free to file future issues under the module in question and explain in the free text.

[andreasplesch]

Thanks, will do.