Put xml:id before xmlns on the manual root element

Required by the AttributeOrderSniff of php/docbook-cs.

Author: lacatoire

manual.xml

@@ -26,7 +26,7 @@
 
 ]>
 
-<set xmlns="http://docbook.org/ns/docbook" version="5.2" xml:id="index" xml:lang="&LANG;" xmlns:xlink="http://www.w3.org/1999/xlink">
+<set xml:id="index" xmlns="http://docbook.org/ns/docbook" version="5.2" xml:lang="&LANG;" xmlns:xlink="http://www.w3.org/1999/xlink">
  <info>
   <title>PHP Extension Documentation</title>
   <abstract>