[css-anchor-position-1][editorial] fix heading level

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -421,8 +421,8 @@ Its syntax is:
 </dl>
 
 
-<h3 id=resolving-spans>
-Resolving <<inset-area-span>>s</h3>
+<h4 id=resolving-spans>
+Resolving <<inset-area-span>>s</h4>
 
 The <dfn export>inset-area grid</dfn> is conceptually a 3x3 grid,
 composed of four grid lines in each axis.