updated notebook and requirements.txt to pin a RAGAS version

[wei-m-teh]

Issue #, if available:

Description of changes:
updated requirements.txt file to use a specific version of RAGAS consistent with the blog post.
Updated notebooks markdown to indicate using python 3 kernel in SageMaker Studio Jupyterlab.

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[rsgrewal-aws]

Please re do this PR following the new layout and the process for creating the required file . We request all files to have a presence on the Cookbook / Recipes Website which is fronting this github repo now. Some of the mandatory sections of the notebook need to include

• Overview.
• What are we demonstrating .
• What use case .
• What will you learn.
• What is the architectural pattern and why we select this. With a diagram
• What are the libraries to install
• What model did we choose and why this model
• Every cell needs to have a markup