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

Author: fantasai

css-overflow-3/Overview.bs

@@ -599,8 +599,7 @@ Indicating Block-Axis Overflow: the 'block-overflow' property</h2>
 		<xmp highlight=css>
 			li {
 				max-lines: 5;
-				block-overflow: ellipsis;
-				block-ellipsis: "… (continued on next page)";
+				block-overflow: "… (continued on next page)";
 			}
 			strong {
 				display: block;