Fix typo

Author: wschin

test/Microsoft.ML.Tests/Scenarios/Api/Estimators/IntrospectiveTraining.cs

@@ -65,7 +65,7 @@ public void FastTreeClassificationIntrospectiveTraining()
             var model = pipeline.Fit(data);
 
             // Extract the learned GBDT model.
-            var treeCollection = ((FastForestBinaryModelParameters)((Internal.Calibration.FeatureWeightsCalibratedPredictor)pred.Model).SubPredictor).TrainedTreeEnsemble;
+            var treeCollection = ((FastTreeBinaryModelParameters)((Internal.Calibration.FeatureWeightsCalibratedPredictor)pred.Model).SubPredictor).TrainedTreeEnsemble;
 
             // Inspect properties in the extracted model.
             Assert.Equal(3, treeCollection.Trees.Count);