Simple consistency improvement on aspect ratio

Resolves #1486

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -564,7 +564,7 @@
 
 For a practical example, an image 800 pixels wide by 400 pixels high has a 2∶1 aspect ratio.
 
-The image's aspect ratio can be determined from the ratio of its height to its width. However, since
+The image's aspect ratio can be determined from the ratio of its width to its height. However, since
 we have a given aspect ratio in mind, it's easier to set the image's width and the aspect ratio, and
 then using this to calculate for its height. This way, we can scale up or down the image by changing
 the image width, and it won't throw off our desired aspect ratio. We do have to make sure that when