Correct wording for ray traversal

Resolves #766

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -490,7 +490,7 @@
 
 I’ll put the “eye” (or camera center if you think of a camera) at $(0,0,0)$. I will have the y-axis
 go up, and the x-axis to the right. In order to respect the convention of a right handed coordinate
-system, into the screen is the negative z-axis. I will traverse the screen from the lower left hand
+system, into the screen is the negative z-axis. I will traverse the screen from the upper left hand
 corner, and use two offset vectors along the screen sides to move the ray endpoint across the
 screen. Note that I do not make the ray direction a unit length vector because I think not doing
 that makes for simpler and slightly faster code.