Unified the syntax of the XML root element (install-guide)

The XML root element of Docbook XML files should match the following
format:

<ELEMENT xmlns="http://docbook.org/ns/docbook"
  xmlns:xi="http://www.w3.org/2001/XInclude"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  version="5.0"
  xml:id="THE_XML_ID_OF_THE_ELEMENT">

Change-Id: I1e0804e2c5021bd78b77483f3156c5b069453555