[css-color-4] close broken <dfn> tag

Author: Peter Linss

css-color/Overview.bs

@@ -1786,7 +1786,7 @@ In adition there is a third, optional descriptor to define the ''rendering inten
 This is only needed if the profile linked to has data for multiple rendering intents.
 
 <pre class="prod">
-	<dfn at-rule 
+	<dfn at-rule >FIX ME: MISSING</dfn>
 </pre>
 
 <h2 id='cmyk-colors'>