[css-nesting-1][editorial] Fix nesting level for changes section, was part of CSSOM

Author: svgeesus

css-nesting-1/Overview.bs

@@ -1339,8 +1339,7 @@ will collapse into a single rule when serialized and parsed again.
 
 <!-- Big Text: changes -->
 
-Changes {#changes}
--------
+<h2 id="changes">Changes</h2>
 
 Significant changes since the
 <a href="https://www.w3.org/TR/2023/WD-css-nesting-1-20230214/">Feb 14, 2023 Working Draft</a>: