fixed error in Ackley

cbx/objectives.py

@@ -494,7 +494,7 @@ def __init__(self, A=20., b=0.2, c=2*np.pi, minimum = None):
         self.A=A
         self.b=b
         self.c=c
-        self.minima = np.array([[[0,0]]]) if minimum is None else minimum
+        self.minima = 0 if minimum is None else minimum
 
     def apply(self, x):
         d = x.shape[-1]