Add dtype torch.double to basic example on website (#2395)

Summary:
<!--
Thank you for sending the PR! We appreciate you spending the time to make BoTorch better.

Help us understand your motivation by explaining why you decided to make this change.

You can learn more about contributing to BoTorch here: https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md
-->

## Motivation

By chance, I encountered that there is no explicit dtype provided in the basic example on the botorch website, which then throws a warning when running the code, which I find kind of annoying. This PR just adds `dtype=torch.double` to the example, as it is also done for somehow the same code example in the `README.md` on github.

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

Yes.

Pull Request resolved: #2395

Test Plan: No additional tests needed.

Reviewed By: Balandat

Differential Revision: D59056278

Pulled By: esantorella

fbshipit-source-id: 45786326d421e687482a038068bc4d6a21e9f84c

Author: jduerholt

website/pages/en/index.js

@@ -118,7 +118,7 @@ from botorch.fit import fit_gpytorch_mll
 from botorch.utils import standardize
 from gpytorch.mlls import ExactMarginalLogLikelihood
 
-train_X = torch.rand(10, 2)
+train_X = torch.rand(10, 2, dtype=torch.double)
 Y = 1 - torch.linalg.norm(train_X - 0.5, dim=-1, keepdim=True)
 Y = Y + 0.1 * torch.randn_like(Y)  # add some noise
 train_Y = standardize(Y)