Upgrade reference docs to DocBook 5

For compatibility with Gradle docbook-reference-plugin, which cannot
handle DocBook 4.

src/reference/docbook/preface.xml

@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE preface PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
- "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
-
-<preface id="preface">
+<preface xmlns="http://docbook.org/ns/docbook" version="5.0"
+    xmlns:xlink="http://www.w3.org/1999/xlink"
+    xmlns:xi="http://www.w3.org/2001/XInclude"
+    xml:id="preface">
   <title>Preface</title>
 
   <para>Developing software applications is hard enough even with good tools
@@ -36,4 +36,4 @@
   have any requests or comments, please post them on the user mailing list or
   on the support forums at <ulink url="http://forum.springsource.org/" />.
   </para>
-</preface>
+</preface>