Unified the syntax of the XML root element (admin-guide-cloud)

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: I95a6f36d6887af952257a800e3359d53ac1f895a
Merge "Unified the syntax of the XML root element (admin-guide-cloud)"