[css-backgrounds-3] Fix error in example.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -157,7 +157,7 @@ background-repeat: no-repeat;
 dropped and the missing values for 'background-origin' and
 'background-repeat' filled in (emphasized for clarity):
 <pre>
-background-image: url(flower.png), url(ball.png), url(grass1.png);
+background-image: url(flower.png), url(ball.png), url(grass.png);
 background-position: center center, 20% 80%, top left;
 background-origin: border-box, content-box<strong>, border-box</strong>;
 background-repeat: no-repeat<strong>, no-repeat, no-repeat</strong>;