fix import error at model inversion attacks notebook

Author: Runhua Xu

notebooks/model_inversion_attacks_mnist.ipynb

@@ -34,7 +34,7 @@
     "seed(123)\n",
     "\n",
     "from art.estimators.classification import KerasClassifier\n",
-    "from art.attacks.inference import MIFace\n",
+    "from art.attacks.inference.model_inversion.mi_face import MIFace\n",
     "from art.utils import load_dataset\n",
     "\n",
     "%matplotlib inline\n",