FB_geometry_metadata: Clarify meaning of "primitiveCount"

[toji]

This is pretty simple, but on my first read of the FB_geometry_metadata extension 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.