Fix Multi-Task GP example in docstring (pytorch#447)

Summary:
Addresses pytorch#446
Pull Request resolved: pytorch#447

Reviewed By: qingfeng10

Differential Revision: D21594459

Pulled By: Balandat

fbshipit-source-id: a3b4d14791175ef67ca28e8d9582e13cc37ab083

Author: Balandat

botorch/models/multitask.py

@@ -68,10 +68,10 @@ def __init__(
         Example:
             >>> X1, X2 = torch.rand(10, 2), torch.rand(20, 2)
             >>> i1, i2 = torch.zeros(10, 1), torch.ones(20, 1)
-            >>> train_X = torch.stack([
+            >>> train_X = torch.cat([
             >>>     torch.cat([X1, i1], -1), torch.cat([X2, i2], -1),
             >>> ])
-            >>> train_Y = torch.cat(f1(X1), f2(X2))
+            >>> train_Y = torch.cat(f1(X1), f2(X2)).unsqueeze(-1)
             >>> model = MultiTaskGP(train_X, train_Y, task_feature=-1)
         """
         self._validate_tensor_args(X=train_X, Y=train_Y)