book1: typo trig "qualities" -> "identities"

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -3457,7 +3457,7 @@
 the water, but when you are close to the surface and looking sideways, the water surface looks like
 a mirror.
 
-We can solve for `sin_theta` using the trigonometric qualities:
+We can solve for `sin_theta` using the trigonometric identities:
 
   $$ \sin\theta  = \sqrt{1 - \cos^2\theta} $$