Merge pull request #736 from antoniogamiz/fix/typo

Fix typo in math expression, close #735

Author: hollasch

books/RayTracingTheRestOfYourLife.html

@@ -327,7 +327,7 @@
 
   $$
   area(C'r, 0, 2) = \int_{0}^{2} C' r dr
-                  = \frac{C'r^2}{2} \biggr|_{r=2}^{r=0}
+                  = \frac{C'r^2}{2} \biggr|_{r=0}^{r=2}
                   = \frac{C' \cdot 2^2}{2} - \frac{C' \cdot 0^2}{2}
                   = 2C'
   $$