[css-lists] Add a space after image bullets.

Author: tabatkins

css-lists/Overview.bs

@@ -500,7 +500,8 @@ Generating the value of the 'content' property</h3>
 	<dl>
 		<dt>an <<image>>
 		<dd>
-			The computed value of the 'content' property is the computed value of the <<image>>.
+			The computed value of the 'content' property is the computed value of the <<image>>,
+			followed by a string containing a single space (U+0020 SPACE).
 
 		<dt>a <<string>>
 		<dd>