Book 3 12.4 missing modification in `ray_color` to sample glass ball

[jianjianh1]

The section changes the sphere class to let us shoot sample ray to the glass ball, but the ray_color function doesn't change any sampling scheme.

Furthermore, it mentions

It took five times as long as sampling the light did for my code. This is probably because those rays that hit the glass are expensive!

So I believe the modification in ray_color is missing.