[css-variables] The dependency graph should be directed

Author: SimonSapin

css-variables/Overview.bs

@@ -270,7 +270,7 @@ Resolving Dependency Cycles</h3>
 	or two or more <a>custom properties</a> each attempt to refer to each other.
 
 	For each element,
-	create an undirected dependency graph,
+	create an directed dependency graph,
 	containing nodes for each <a>custom property</a>.
 	If the value of a <a>custom property</a> <var>prop</var>
 	contains a ''var()'' function referring to the property <var>var</var>