[css-overflow-4] Fix markup typo

Author: frivoal

css-overflow-4/Overview.bs

@@ -299,7 +299,7 @@ The values of this property have the following meaning:
 	<dt><dfn>stable</dfn>
 	<dd>The <a>scrollbar gutter</a> is present
 	for <a>classic scrollbars</a>
-	when 'overflow' is ''overflow/hidden', ''overflow/scroll'', or ''overflow/auto'',
+	when 'overflow' is ''overflow/hidden'', ''overflow/scroll'', or ''overflow/auto'',
 	regardless of whether the box is actually overflowing.
 	<a>Overlay scrollbars</a> do not consume space.