fixed CrossValidator.avgMetric calculation

Author: pkch

python/pyspark/ml/tuning.py

@@ -234,7 +234,7 @@ def _fit(self, dataset):
                 model = est.fit(train, epm[j])
                 # TODO: duplicate evaluator to take extra params from input
                 metric = eva.evaluate(model.transform(validation, epm[j]))
-                metrics[j] += metric
+                metrics[j] += metric/nFolds
 
         if eva.isLargerBetter():
             bestIndex = np.argmax(metrics)