Fix Web IDL syntax encountered by Reffy (#2661)

Author: foolip

css-fonts-4/Overview.bs

@@ -3805,7 +3805,7 @@ The <code>CSSFontFeatureValuesRule</code> interface</h3>
 
 <pre class='idl'>partial interface CSSRule {
   const unsigned short FONT_FEATURE_VALUES_RULE = 14;
-}</pre>
+};</pre>
 
 <p>The <dfn>CSSFontFeatureValuesRule</dfn> interface represents a <code>@font-feature-values</code> rule.</p>