[css-nesting-1][editorial] Fix markup

Author: tabatkins

css-nesting-1/Overview.bs

@@ -757,7 +757,8 @@ Nesting Other At-Rules {#conditionals}
 	to remove the wrapping rule
 	when the first child rule is <code>& {...}</code>.
 
-### Nested ''@scope'' Rules {#nesting-at-scope}
+<h4 id=nesting-at-scope>
+Nested ''@scope'' Rules</h4>
 
 	When the ''@scope'' rule is a [=nested group rule=],
 	an ''&'' in the <<scope-start>> selector