Update MultiTaskGP Docstring (#3307)

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## Motivation

This PR fixes a wrong docstring regarding the noise prior of the multitask gp.

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

Yes.

Pull Request resolved: #3307

Test Plan: Only docs.

Reviewed By: SebastianAment

Differential Revision: D105971096

Pulled By: saitcakmak

fbshipit-source-id: 7c0f7e747290a39fe3f2e328a308976edd497dd5

Author: jduerholt

botorch/models/multitask.py

@@ -646,7 +646,7 @@ def __init__(
             train_X: A ``batch_shape x n x d`` tensor of training features.
             train_Y: A ``batch_shape x n x m`` tensor of training observations.
             likelihood: A ``MultitaskGaussianLikelihood``. If omitted, uses a
-                ``MultitaskGaussianLikelihood`` with a ``GammaPrior(1.1, 0.05)``
+                ``MultitaskGaussianLikelihood`` with a ``LogNormalPrior(-4, 1)``
                 noise prior.
             data_covar_module: The module computing the covariance (Kernel) matrix
                 in data space. If omitted, uses an ``RBFKernel``.