Fix typo

Change `add` to `and` in Book 2 Section 5.1
Resolves #1683

Author: swigof

books/RayTracingTheNextWeek.html

@@ -2069,7 +2069,7 @@
 </div>
 
 <div class='together'>
-Add the hashing does scramble as hoped:
+And the hashing does scramble as hoped:
 
   ![<span class='num'>Image 9:</span> Hashed random texture
   ](../images/img-2.09-hash-random.png class='pixel')