Fixes the incorrect color formulation. Resolves #199

Author: Trevor Black

books/RayTracingTheRestOfYourLife.html

@@ -654,7 +654,7 @@
 If we sample using the same pdf, so $p(direction) = cos(\theta) / \pi$, the numerator and
 denominator cancel out and we get:
 
-  $$ Color = A * s(direction) $$
+  $$ Color = A * color(direction) $$
 
 This is exactly what we had in our original color() function! But we need to generalize now so we
 can send extra rays in important directions such as toward the lights.