[css-overflow] fix typo

Author: frivoal

css-overflow-4/Overview.bs

@@ -1028,7 +1028,7 @@ Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>
 		and thus cannot [=hang=].
 
 		<wpt>
-			block-ellipsis-034.htm
+			block-ellipsis-034.html
 		</wpt>