Add missing extract class on dashed_attribute

Author: saschanaz

cssom/Overview.bs

@@ -2245,7 +2245,7 @@ For each CSS property <var>property</var> that is a <a>supported CSS property</a
 except for properties that have no "<code>-</code>" (U+002D) in the property name,
 the following partial interface applies where <var>dashed attribute</var> is <var>property</var>.
 
-<pre class="idl">
+<pre class="idl extract">
 partial interface CSSStyleDeclaration {
   [CEReactions, TreatNullAs=EmptyString] attribute CSSOMString _<var>dashed_attribute</var>;
 };