[css-anchor-position-1] Fix anchor-center section number (w3c#9346)

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -643,8 +643,8 @@ This has two effects:
 ██     ██          ██████  ████████ ██    ██    ██    ████████ ██     ██
 -->
 
-<h2 id="anchor-center">
-Centering on the Anchor: the ''anchor-center'' value</h2>
+Centering on the Anchor: the ''anchor-center'' value {#anchor-center}
+------------------------------------------------------------------------
 
 <pre class=propdef>
 Name: justify-self, align-self, justify-items, align-items