Update xml namespace

The following error message is displayed when using the file as-is:

```shell

phpdox:
     [exec] phpDox 0.7.0-dirty - Copyright (C) 2010 - 2014 by Arne Blankerts
     [exec]
     [exec]
     [exec] An error occured while trying to load the configuration file:
     [exec] 	File './phpdox.xml' uses an outdated xml namespace. Please update the xmlns to 'http://xml.phpdox.net/config'
     [exec]
     [exec] Using --skel might get you started.
     [exec]
     [exec] Result: 3

```