Use correct var name in the triangle mesh sample

PR #3 pointed out some typos in the "how to load a triangle mesh" sample code. One of them was an undefined variable, `propIdx`, when checking whether we need to triangulate the face data. I noticed after merging the PR that it was attempting to work around a typo I'd made in the variable name: it should actually have been `indexes[0]` (or `*indexes`) instead. I'm just fixing that up here.

README.md

@@ -153,8 +153,7 @@ TriMesh* load_trimesh_from_ply(const char* filename)
       gotVerts = true;
     }
     else if (reader.element_is(miniply::kPLYFaceElement) && reader.load_element() && reader.find_indices(indexes)) {
-      uint32_t propIdx;
-      bool polys = reader.requires_triangulation(propIdx);
+      bool polys = reader.requires_triangulation(indexes[0]);
       if (polys && !gotVerts) {
         fprintf(stderr, "Error: need vertex positions to triangulate faces.\n");
         break;