[css-overflow-3] Fix markup

Author: fantasai

css-overflow-3/Overview.bs

@@ -322,8 +322,9 @@ Scrolling Overflow</h3>
 
 	A box’s [=overflow=] can be visible or clipped.
 	CSS also allows a box to be a <dfn export>scroll container</dfn>
-	that allows clipped parts of its scrollable overflow area to be scrolled into view.
-	The visual viewport of a <a>scroll container</a>
+	that allows clipped parts of its [=scrollable overflow area=]
+	to be scrolled into view.
+	The visual “viewport” of a <a>scroll container</a>
 	(through which the <a>scrollable overflow area</a> can be viewed)
 	coincides with its padding box,
 	and is called the <dfn export>scrollport</dfn>.