Fix typo "arbirary"

Resolves #1589

Author: hollasch

CHANGELOG.md

@@ -14,6 +14,7 @@ Change Log / Ray Tracing in One Weekend
 ### The Next Week
 
 ### The Rest of Your Life
+  - Fix    -- Fix typo of "arbitrary" (#1589)
 
 
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books/RayTracingTheRestOfYourLife.html

@@ -2472,9 +2472,9 @@
 Sampling Lights Directly
 ====================================================================================================
 The problem with sampling uniformly over all directions is that lights are no more likely to be
-sampled than any arbirary or unimportant direction. We could use shadow rays to solve for the direct
-lighting at any given point. Instead, I’ll just use a PDF that sends more rays to the light. We can
-then turn around and change that PDF to send more rays in whatever direction we want.
+sampled than any arbitrary or unimportant direction. We could use shadow rays to solve for the
+direct lighting at any given point. Instead, I’ll just use a PDF that sends more rays to the light.
+We can then turn around and change that PDF to send more rays in whatever direction we want.
 
 It’s really easy to pick a random direction toward the light; just pick a random point on the light
 and send a ray in that direction. But we'll need to know the PDF, $p(\omega)$, so that we're not