[css-overflow-4][editorial] Fix markup error

Author: fantasai

css-overflow-4/Overview.bs

@@ -1703,7 +1703,7 @@ showing in which case space is reserved for the <a>scrollbar gutter</a>.
 		<th rowspan=2>'overflow'
 		<th rowspan=2>'scrollbar-gutter'
 		<th colspan=2>Classic scrollbars
-		<th colspan=2 rowspan=2>Overlay scrollbars <small>(whether overflowing or not)</small>
+		<th rowspan=2>Overlay scrollbars <small>(whether overflowing or not)</small>
 	<tr>
 		<th>Overflowing
 		<th>Not overflowing