Update multitask.py (#2621)

Summary:
fix bug in example

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## Motivation

Improve documentation.

### Have you read the [Contributing Guidelines on pull requests](https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

Yes

Pull Request resolved: #2621

Test Plan:
n/a

## Related PRs

n/a

Reviewed By: Balandat

Differential Revision: D65878578

Pulled By: saitcakmak

fbshipit-source-id: 05c222f81952974d3f4e11dc6f16d775e6ed23c2

botorch/models/multitask.py

@@ -195,7 +195,7 @@ def __init__(
             >>> train_X = torch.cat([
             >>>     torch.cat([X1, i1], -1), torch.cat([X2, i2], -1),
             >>> ])
-            >>> train_Y = torch.cat(f1(X1), f2(X2)).unsqueeze(-1)
+            >>> train_Y = torch.cat([f1(X1), f2(X2)]).unsqueeze(-1)
             >>> model = MultiTaskGP(train_X, train_Y, task_feature=-1)
         """
         with torch.no_grad():