Documentation: Update git repository URL for Smack userland tools

The userland git repository has been moved to a new place.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Rob Landley <rob@landley.net>
Cc: linux-security-module@vger.kernel.org
Cc: lkml@vger.kernel.org