[css-contain-2] Fix markup error

Author: frivoal

css-contain-2/Overview.bs

@@ -1540,7 +1540,7 @@ Answers are provided below.
 
 This appendix is <em>informative</em>.
 
-<h3 id="l1-changes">
+<h3 id="changes-since-2019-11-11">
 Changes from <a href="https://www.w3.org/TR/2019/WD-css-contain-2-20191111/">2019-11-11 Working Draft</a>
 </h3>