fix Singleton.__call__ error

fix Singleton.__call__ error

mininet/log.py

@@ -69,7 +69,7 @@ def __init__( cls, name, bases, dict_ ):
     def __call__( cls, *args, **kw ):
         if cls.instance is None:
             cls.instance = super( Singleton, cls ).__call__( *args, **kw )
-            return cls.instance
+        return cls.instance
 
 
 class MininetLogger( Logger, object ):