[cssom] Change the ID of a heading to avoid clashes.

Author: tabatkins

cssom/Overview.bs

@@ -669,8 +669,8 @@ finally return <var>s</var>:
 </dl>
 
 
-CSS {#css}
-==========
+CSS {#css-object-model}
+=======================
 
 
 CSS Style Sheets {#css-style-sheets}