Update migration challenge PyTorch instruction notebook:

[furixturi]

Correct S3 data folder name in TODO comment

Issue #, if available: N/A

Description of changes: In /migration_challenge/pytorch_mnist/Instructions.ipynb, the TODO code cell under Data Input("Channels") Configuration says

# The data can be found in: "s3://{bucket_name}/mnist/train" and "s3://{bucket_name}/mnist/test"

But as set up in earlier code cells, the folder names should be "s3://{bucket_name}/mnist/training" and "s3://{bucket_name}/mnist/testing"

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