[css-values-4] Fix missing parens.

Author: fantasai

css-values-4/Overview.bs

@@ -1879,7 +1879,7 @@ The Large, Small, and Dynamic Viewport Sizes</h5>
 	Additionally,
 	UAs may have some dynamically-shown interfaces which intentionally overlay content
 	and do not cause any shifts in layout
-	(and therefore have no effect on any of the [=viewport-percentage lengths=];
+	(and therefore have no effect on any of the [=viewport-percentage lengths=]);
 	typically on-screen keyboards will fit into this category.
 
 	In all cases, scrollbars are assumed not to exist.