Upgrade reference docs to DocBook 5

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

src/reference/docbook/swf-sidebar.xml

@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE sidebar PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
- "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
-
-<sidebar>
+<sidebar 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="spring-web-flow-sidebar">
   <title>Spring Web Flow</title>
 
   <para>Spring Web Flow (SWF) aims to be the best solution for the management
@@ -21,4 +21,4 @@
   <para>For more information about SWF, consult the
   <ulink url="http://www.springframework.org/webflow">Spring Web Flow website</ulink>.
   </para>
-</sidebar>
+</sidebar>