FB_geometry_metadata: Clarify meaning of "primitiveCount"

This is pretty simple, but on my first read of the [FB_geometry_metadata extension](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/FB_geometry_metadata) I thought that the `primitiveCount` referred to the number of Mesh Primitives in the scene. The example that's given indicating a `vertexCount` of 28727 and a `primitiveCount` of 15451, however, suggests that the intended meaning is the number of geometric primitives (triangles, lines, etc) that the scene contains. (Unless the example given represents a scene with a very high number of Primitives, each averaging 1.8 vertices, which seems... unlikely.)

In any case, because "Primitive" is used by the glTF spec to refer to something other than the geometric primitives that GPU APIs deal with, it's worth clarifying in the extension which meaning is intended to be applied here.

Open the FB_geometry_metadata extension linked in the issue and read the definition of primitiveCount alongside the vertexCount and primitiveCount example. Clarify the term's intended meaning in the extension text so it cannot be confused with glTF Mesh Primitives; done means the distinction is explicit.

Written by the indexing model from the issue text.