Book 2: fix typo

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -495,7 +495,7 @@
   ![Figure [ray-slab-interval]: Ray-slab $t$-interval overlap
   ](../images/fig-2.04-ray-slab-interval.jpg)
 
-In this figure, the upper ray intervals to not overlap, so we know the ray does _not_ hit the 2D box
+In this figure, the upper ray intervals do not overlap, so we know the ray does _not_ hit the 2D box
 bounded by the green and blue planes. The lower ray intervals _do_ overlap, so we know the lower ray
 _does_ hit the bounded box.