Fix FastTree regression samples (#4573)

Author: najeeb-kazmi

docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/FastTree.cs

@@ -27,7 +27,7 @@ public static void Example()
             var trainingData = mlContext.Data.LoadFromEnumerable(dataPoints);
 
             // Define the trainer.
-            var pipeline = mlContext.Regression.Trainers.FastForest(
+            var pipeline = mlContext.Regression.Trainers.FastTree(
                 labelColumnName: nameof(DataPoint.Label),
                 featureColumnName: nameof(DataPoint.Features));
 

docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/FastTree.tt

@@ -7,7 +7,7 @@ string ClassHeader = @"// This example requires installation of additional NuGet
 
 string ClassName="FastTreeRegression";
 string ExtraUsing = null;
-string Trainer = @"FastForest(
+string Trainer = @"FastTree(
                 labelColumnName: nameof(DataPoint.Label),
                 featureColumnName: nameof(DataPoint.Features))";
 string TrainerOptions = null;

docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/FastTreeTweedie.cs

@@ -27,7 +27,7 @@ public static void Example()
             var trainingData = mlContext.Data.LoadFromEnumerable(dataPoints);
 
             // Define the trainer.
-            var pipeline = mlContext.Regression.Trainers.FastForest(
+            var pipeline = mlContext.Regression.Trainers.FastTreeTweedie(
                 labelColumnName: nameof(DataPoint.Label),
                 featureColumnName: nameof(DataPoint.Features));
 

docs/samples/Microsoft.ML.Samples/Dynamic/Trainers/Regression/FastTreeTweedie.tt

@@ -7,7 +7,7 @@ string ClassHeader = @"// This example requires installation of additional NuGet
 
 string ClassName="FastTreeTweedieRegression";
 string ExtraUsing = null;
-string Trainer = @"FastForest(
+string Trainer = @"FastTreeTweedie(
                 labelColumnName: nameof(DataPoint.Label),
                 featureColumnName: nameof(DataPoint.Features))";
 string TrainerOptions = null;