[css-lists] Clarify anaphora.

Author: tabatkins

css-lists-3/Overview.bs

@@ -103,7 +103,7 @@ The ''::marker'' Pseudo-Element</h3>
 
 	The <a>marker box</a> is generated by
 	the ''::marker'' pseudo-element of a <a>list item</a>
-	as its first child,
+	as the [=list item’s=] first child,
 	before the ''::before'' pseudo-element
 	(if it exists on the element).
 	It is filled with content