[css-values-5][editorial] fix typo

Author: fantasai

css-values-5/Overview.bs

@@ -2975,7 +2975,7 @@ Tree Counting Functions: the ''sibling-count()'' and ''sibling-index()'' notatio
 
 
 	A [=tree-counting function=]
-	is is a type of  [=tree-scoped-reference/loosely-matched=] [=tree-scoped reference=],
+	is a type of  [=tree-scoped-reference/loosely-matched=] [=tree-scoped reference=],
 	which is resolved as if the element were given an automatic [=tree-scoped name=]
 	that matches an identical automatic name on the [=tree-scoped reference=]
 	of the [=tree-counting function=].