fix a/an typo.

Author: zhangenming

css-contain-2/Overview.bs

@@ -592,7 +592,7 @@ Size Containment</h3>
 				contain-size-grid-001.html
 				</wpt>
 
-				<a>Replaced elements</a> must be treated as having an [=natural dimensions|natural=] width and height of 0
+				<a>Replaced elements</a> must be treated as having a [=natural dimensions|natural=] width and height of 0
 				and no [=natural aspect ratio=].
 
 				Note: Size containment only suppresses the [=natural aspect ratio=],