[css-overflow-5][editorial] fix heading level

Author: tabatkins

css-overflow-5/Overview.bs

@@ -340,8 +340,8 @@ Selecting The Active Scroll Marker: the '':target-current'' pseudo-class</h4>
 	the next tabindex-ordered focus navigation will focus the [=scroll target=] if it is focusable,
 	otherwise, it will find the next focusable element from this <var>element</var> as though it were focused.
 
-<h2 id="scroll-buttons">
-Scroll Buttons</h2>
+<h3 id="scroll-buttons">
+Scroll Buttons</h3>
 
 The <dfn>::scroll-button( <<scroll-button-direction>> )</dfn> pseudo-elements
 are generated on [=scroll containers=]