Fix gradient image description

Resolves #1244

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -148,8 +148,8 @@
      before we print them out.
 
   5. Red goes from fully off (black) to fully on (bright red) from left to right, and green goes
-     from fully off at the top to black at the bottom. Adding red and green light together make
-     yellow so we should expect the bottom right corner to be yellow.
+     from fully off at the top (black) to fully on at the bottom (bright green). Adding red and
+     green light together make yellow so we should expect the bottom right corner to be yellow.
 
 
 Creating an Image File