fix: clarify grammar

Author: vwkd

css-backgrounds-3/Overview.bs

@@ -1008,7 +1008,7 @@ body {
 <p>The following example rounds the height of the image to 33.3%, up
 from the specified value of 30%. At 30%, three images would fit
 entirely and a fourth only partially. After rounding, three images
-fit. The width of the image is 20% of the background area width and is
+fit. The width of the image is 20% of the background positioning area width and is
 not rounded.
 
 <pre>