[css-anchor-position] fix markup

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1025,7 +1025,7 @@ we define:
 	depend on an anchor's scroll position,
 	the positioned element must be inside of the same scroll container,
 	so the two elements are never scrolled relative to each other at all.
-</div>
+</details>
 
 Validity {#anchor-valid}
 --------