[Book 3] Remove highlights for empty lines in listing 6

Addresses follow-up comments on PR #1400

Author: armansito

books/RayTracingTheRestOfYourLife.html

@@ -347,9 +347,8 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
             sqrt_spp = int(sqrt(samples_per_pixel));
             recip_sqrt_spp = 1.0 / sqrt_spp;
-
-
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
+
             // Calculate the u,v,w unit basis vectors for the camera coordinate frame.
             w = unit_vector(lookfrom - lookat);
             ...