[css-contain-1] Fix WPT file name

Author: xfq

css-contain-1/Overview.bs

@@ -467,9 +467,9 @@ Layout Containment</h3>
 
 		<wpt>
 		contain-layout-baseline-001.html
-		css/css-contain/contain-layout-button-001.html
-		css/css-contain/contain-layout-flexbox-001.html
-		css/css-contain/contain-layout-grid-001.html
+		contain-layout-button-001.html
+		contain-layout-flexbox-001.html
+		contain-layout-grid-001.html
 		</wpt>
 
 	Possible optimizations that can be enabled by <a>layout containment</a> include (but are not limited to):