[css-sizing-4] Fix syntax error in example.

Author: fantasai

css-sizing-4/Overview.bs

@@ -157,9 +157,9 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 			  height=315>
 		</pre>
 		<pre highlight="css">
-			@supports (aspect-ratio: attr(width px) / 1) {
+			@supports (aspect-ratio: attr(width) / 1) {
 			  iframe {
-			    aspect-ratio: attr(width px) / attr(height px);
+			    aspect-ratio: attr(width) / attr(height);
 			    width: 100%;
 			    height: auto;
 			  }