[Content Improvement] Update instance types in pytorch_smdataparallel_mnist_demo

**Link to the notebook**
[pytorch_smdataparallel_mnist_demo](https://github.com/aws/amazon-sagemaker-examples/blob/main/training/distributed_training/pytorch/data_parallel/mnist/pytorch_smdataparallel_mnist_demo.ipynb)

**What aspects of the notebook can be improved?**
This notebook is not working anymore with `ml.p3dn.24xlarge` instances : `botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the CreateTrainingJob operation: Unsupported instance type ml.p3dn.24xlarge`

**What are your suggestions?**
Don't suggest `ml.p3dn.24xlarge` as a recommended instance

Open training/distributed_training/pytorch/data_parallel/mnist/pytorch_smdataparallel_mnist_demo.ipynb and search for the ml.p3dn.24xlarge recommendation. Review the surrounding SageMaker training configuration, update the notebook so it no longer recommends that unsupported instance type, and run the affected notebook cells to confirm the example remains usable.

Written by the indexing model from the issue text.