[css-color-5][editorial] Fix mismatched tags

Author: kbabbitt

css-color-5/Overview.bs

@@ -2866,7 +2866,7 @@ or any other color or monochrome output device which has been characterized.
 
 		<pre class="lang-css">
 			background-image: light-dark(url(my-light-image.png), image(transparent));
-		<pre>
+		</pre>
 	</div>