add doctype to theme demo

Author: pgfearo

sample/theme-demo.xsl

@@ -1,3 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE example [
+ <!ENTITY company "Microsoft">
+]>
+
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
  version="3.0"
  expand-text="yes">