Book 3.10.1: `camera::ray_color()` switches from `double pdf` to `double pdf_val` halfway through

[trevordblack]

Pick one of the above and make it consistent.

[hollasch]

This is a local variable in camera::ray_color(). Also confusing versus hittable::pdf_value().

[hollasch]

pdf clashes with the class name. pdf_value clashes with hittable::pdf_value(), though that should be ok as a local name.