[css-grid-1] Export 'grid-level'

Author: fantasai

css-grid-1/Overview.bs

@@ -1050,7 +1050,7 @@ Grid Items</h2>
 	Grid Item Display</h3>
 
 	A <a>grid item</a> [=establishes an independent formatting context=] for its contents.
-	However, grid items are <dfn>grid-level</dfn> boxes, not block-level boxes:
+	However, grid items are <dfn export>grid-level</dfn> boxes, not block-level boxes:
 	they participate in their container's <a>grid formatting context</a>,
 	not in a block formatting context.