Correct book 3 listing 20, geom type + unit vec

`on_light` is a point, not a vector. The code also used obsolete
vec3 method function `make_unit_vector`.

Resolves #606

Author: hollasch

CHANGELOG.md

@@ -6,6 +6,7 @@ Change Log -- Ray Tracing in One Weekend
 ### _The Rest of Your Life_
   - Fix: Missing closing parenthesis in listing 10 (#603)
   - Fix: Tiny improvements to the lambertian::scatter() development (#604)
+  - Fix: Correct geometry type and unit vector method in `ray_color()`, listing 20 (#606)
 
 
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books/RayTracingTheRestOfYourLife.html

@@ -1391,10 +1391,10 @@
             return emitted;
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
-        auto on_light = vec3(random_double(213,343), 554, random_double(227,332));
+        auto on_light = point3(random_double(213,343), 554, random_double(227,332));
         auto to_light = on_light - rec.p;
         auto distance_squared = to_light.length_squared();
-        to_light.make_unit_vector();
+        to_light = unit_vector(to_light);
 
         if (dot(to_light, rec.normal) < 0)
             return emitted;