WinRT app manifest: update namespace prefix

If the prefix isn't "m2" for the 2013 namespace, Visual Studio Update 2
won't open the document in the designer view. Even though there is
nothing technically wrong with the way it's currently done, change it to
"m2" to keep VS happy.

Change-Id: I62721114610de5396eb507828b39db89c1e96b1a
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>