Changed math in The Rest Of Your Life for clarity. Resolves RayTracing#378

Author: trevordblack

books/RayTracingTheRestOfYourLife.html

@@ -308,7 +308,8 @@
 
   $$
   area(C'r, 0, 2) = \int_{0}^{2} C' r dr
-                  = \frac{C'r^2}{2}(2) - \frac{C'r^2}{2}(0)
+                  = \frac{C'r^2}{2} \biggr|_{r=2}^{r=0}
+                  = \frac{C' \cdot 2^2}{2} - \frac{C' \cdot 0^2}{2}
                   = 2C'
   $$