[css-overflow-3] Distinguish string from box construct.

Author: fantasai

css-overflow-3/Overview.bs

@@ -632,7 +632,7 @@ Indicating Block-Axis Overflow: the 'block-overflow' property</h2>
 	</div>
 
 	When 'block-overflow' is not ''block-overflow/clip'',
-	the <a>block overflow ellipsis</a>
+	the <a>block overflow ellipsis</a> string
 	is wrapped in an anonymous inline
 	and placed at the end of the line box
 	as a direct child of the <a>block container</a>’s <a>root inline box</a>,