Wrap example output in CDATA (#2736)

Author: timacdonald

chapters/tutorial.xml

@@ -232,7 +232,9 @@ echo $_SERVER['HTTP_USER_AGENT'];
      A sample output of this script may be:
     </para>
     <screen role="html">
+<![CDATA[
 Mozilla/5.0 (Linux) Firefox/112.0
+]]>
     </screen>
    </example>
    </para>