Fix bad typo in xmlns for DocBook 5

This typo ("mlns" instead of "xmlns") was bad as it didn't
bind the DocBook document into the DocBook 5 namespace.
As such, it is under no namespace and considered DocBook 4.

xml/MAIN.susestorage.xml

@@ -9,7 +9,7 @@
 ]>
 <!-- Converted by suse-upgrade version 1.1 -->
 <set version="5.0" xml:lang="en"
- mlns="http://docbook.org/ns/docbook"
+ xmlns="http://docbook.org/ns/docbook"
  xmlns:its="http://www.w3.org/2005/11/its"
  xmlns:xi="http://www.w3.org/2001/XInclude"
  xmlns:xlink="http://www.w3.org/1999/xlink">