[css-grid] Fix heading

Author: tabatkins

css-grid/Overview.bs

@@ -1089,7 +1089,7 @@ Track Sizing: the 'grid-template-rows' and 'grid-template-columns' properties</h
 -->
 
 <h4 id='named-lines'>
-Named Grid Lines: the <css>(<<custom-ident>>*)</css> syntax</h4>
+Named Grid Lines: the <css>[<<custom-ident>>*]</css> syntax</h4>
 
 	While <a>grid lines</a> can always be referred to by their numerical index,
 	<dfn export lt="named line">named lines</dfn>