fix <mark> raw source to match live example

#12956 errata

docs/_includes/css/type.html

@@ -112,7 +112,7 @@
     <p>You can use the mark tag to <mark>highlight</mark> text.</p>
   </div>
 {% highlight html %}
-<mark>This line of text is meant to be treated as deleted text.</mark>
+You can use the mark tag to <mark>highlight</mark> text.
 {% endhighlight %}