[css-position-3] Fix heading level.

Author: fantasai

css-position-3/Overview.bs

@@ -610,8 +610,8 @@ Absolute positioning</h3>
 	Though absolutely positioned boxes may have margins,
 	those margins do not <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a> with any other margins.
 
-<h2 id="breaking">
-Fragmenting Absolutely-positioned Elements</h2>
+<h4 id="breaking">
+Fragmenting Absolutely-positioned Elements</h4>
 
 	In paged media, an absolutely positioned element is positioned relative to its
 	containing block ignoring any page breaks (as if the document were continuous). The