[css-backgrounds-3][editorial] Markup fix.

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -2482,7 +2482,7 @@ Border Image Shorthand: the 'border-image' property</h3>
 
 	Each shadow is given as a <<shadow>>,
 	represented by 2-4 length values, an optional color, and an optional ''box-shadow/inset'' keyword.
-	Omitted lengths are 0; omitted colors default to currentColor.
+	Omitted lengths are 0; omitted colors default to ''currentColor''.
 	<pre class=prod>
 	<dfn><<shadow>></dfn> = <<color>>? &amp;&amp; [<<length>>{2} <<length [0,&infin;]>>? <<length>>?] &amp;&amp; inset?</pre>