Resolves RayTracing#194

Author: Trevor Black

books/RayTracingTheRestOfYourLife.html

@@ -944,7 +944,7 @@
             float r2 = random_double();
             float x = cos(2*M_PI*r1)*2*sqrt(r2*(1-r2));
             float y = sin(2*M_PI*r1)*2*sqrt(r2*(1-r2));
-            float z = 1 - r2;
+            float z = 1 - 2*r2;
 
             std::cout << x << " " << y << " " << z << "\n";
         }