[css-values-3] Fix markup

Author: fantasai

css-values-3/Overview.bs

@@ -686,7 +686,7 @@ Numeric Data Types</h2>
 	(e.g. [[css-grid-1]] introduces ''fr'' units)
 	whose usage is more localized.
 
-<h2 id="numeric-ranges">
+<h3 id="numeric-ranges">
 Range Restrictions and Range Definition Notation</h3>
 
 	Properties can restrict numeric values to some range.