Fix typo by adding missing word

Fixes a typo in Book 2, Listing 53, by adding a missing `if` in the
comment.
Resolves #1560

Author: FineFindus

books/RayTracingTheNextWeek.html

@@ -3038,7 +3038,7 @@
 
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
-            // Determine the hit point lies within the planar shape using its plane coordinates.
+            // Determine if the hit point lies within the planar shape using its plane coordinates.
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
             auto intersection = r.at(t);
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight

src/TheNextWeek/quad.h

@@ -49,7 +49,7 @@ class quad : public hittable {
         if (!ray_t.contains(t))
             return false;
 
-        // Determine the hit point lies within the planar shape using its plane coordinates.
+        // Determine if the hit point lies within the planar shape using its plane coordinates.
         auto intersection = r.at(t);
         vec3 planar_hitpt_vector = intersection - Q;
         auto alpha = dot(w, cross(planar_hitpt_vector, v));

src/TheRestOfYourLife/quad.h

@@ -51,7 +51,7 @@ class quad : public hittable {
         if (!ray_t.contains(t))
             return false;
 
-        // Determine the hit point lies within the planar shape using its plane coordinates.
+        // Determine if the hit point lies within the planar shape using its plane coordinates.
         auto intersection = r.at(t);
         vec3 planar_hitpt_vector = intersection - Q;
         auto alpha = dot(w, cross(planar_hitpt_vector, v));