[css-grid-2] Fix link.

Author: fantasai

css-grid-2/Overview.bs

@@ -212,7 +212,7 @@ Characteristics of a Subgrid Item</h3>
 		<li id="subgrid-area-inheritance">
 			When a <a>subgrid</a> overlaps a <a>named grid area</a> in its parent
 			that was created by a 'grid-template-areas' property declaration,
-			<a>implicit line names</a> are assigned
+			[=implicitly-created line names=] are assigned
 			to represent the parent’s <a>named grid area</a>
 			within the <a>subgrid</a>.