fix notation in simple_mesh example

examples/simple_mesh/main.cpp

@@ -21,7 +21,7 @@ main()
 	rs->render_groups = renAllocateGroup(1, sw);
 	rs->render_group_count = 1;
 
-	entity& e = rs->render_groups->entities[0];
+	entity& e = rs->render_groups[0].entities[0];
 	// manually construct a simple mesh
 	simple_mesh mesh = {0};
 	mesh.num_vertices = 4;