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

Author: tabatkins

css-values-5/Overview.bs

@@ -2804,7 +2804,7 @@ Tree Counting Functions: the ''sibling-count()'' and ''sibling-index()'' notatio
 	</div>
 
 	Note: These functions, to match [=selectors=] like '':nth-child()'', operate on the DOM tree,
-	rather than the [=flat tree] like most CSS values do.
+	rather than the [=flat tree=] like most CSS values do.
 	They may, in the future, have variants that support counting [=flat tree=] siblings.
 
 	Note: These functions may, in the future,