Address `material::scatter()` function mismatch between book and source

[hollasch]

• Book two has a material::scatter() method that takes four arguments.
• Book three text has a material::scatter() method that adds a fifth argument.
• Book three source has a material::scatter() method that takes three arguments.

Also note that the original isotropic::scatter() method in book three took four arguments, so the source code was actually invoking the default material::scatter() method, so that image is probably wrong.