Refactor exploration notebook to use open dataset

The [exploration notebook](https://github.com/aws-samples/amazon-personalize-samples/blob/c07042dc14d4094d828bb48f384645378e1d4473/next_steps/core_use_cases/user_personalization/user-personalization-with-exploration.ipynb) currently references source dataset from an S3 bucket that no longer exists. The notebook should be refactored to use a different public dataset such as MovieLens.