Add the missing rect to listing 54

Resolves #479

Author: oxine

books/RayTracingTheNextWeek.html

@@ -2214,7 +2214,8 @@
         objects.add(make_shared<yz_rect>(0, 555, 0, 555, 0, red));
         objects.add(make_shared<xz_rect>(213, 343, 227, 332, 554, light));
         objects.add(make_shared<xz_rect>(0, 555, 0, 555, 0, white));
-        objects.add(make_shared<xy_rect>(0, 555, 0, 555, 555, white));
+        objects.add(make_shared<xz_rect>(0, 555, 0, 555, 555, white));
+        objects.add(make_shared<xy_rect>(0, 555, 0, 555, 555, white));     
 
         return objects;
     }