Fix typo: "pixrel"

See #976

Author: hollasch

books/RayTracingTheRestOfYourLife.html

@@ -337,7 +337,7 @@
             std::cout << "P3\n" << image_width << " " << image_height << "\n255\n";
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
-            int sqrt_spp = int(sqrt(samples_per_pixrel));
+            int sqrt_spp = int(sqrt(samples_per_pixel));
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
             for (int j = image_height-1; j >= 0; --j) {
                 std::clog << "\rScanlines remaining: " << j << ' ' << std::flush;