[css-position-3][editorial] Move heading up one level

Author: tabatkins

css-position-3/Overview.bs

@@ -973,8 +973,8 @@ Self-Alignment of Absolutely Positioned Boxes</h3>
 	its alignment is adjusted to minimize overflow
 	as specified in [[css-align-3#auto-safety-position]].
 
-<h4 id="abspos-breaking" oldids="breaking">
-Fragmenting Absolutely-positioned Elements</h4>
+<h3 id="abspos-breaking" oldids="breaking">
+Fragmenting Absolutely-positioned Elements</h3>
 
 	In a [=fragmented flow=], an [=absolutely positioned box=]
 	is positioned relative to its [=containing block=]