Fix grammatical error

Add missing `that` in Book 2 Section 3.9
Resolves #1675

Author: swigof

books/RayTracingTheNextWeek.html

@@ -1011,9 +1011,9 @@
 The Box Comparison Functions
 -----------------------------
 Now we need to implement the box comparison functions, used by `std::sort()`. To do this, create a
-generic comparator returns true if the first argument is less than the second, given an additional
-axis index argument. Then define axis-specific comparison functions that use the generic comparison
-function.
+generic comparator that returns true if the first argument is less than the second, given an 
+additional axis index argument. Then define axis-specific comparison functions that use the generic 
+comparison function.
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class bvh_node : public hittable {