Clarify language around datatype casting and delete empty cell in Polars lazy execution tutorial.

[debajyotid2]

📝 Summary

This PR addresses @etrotta's comments in the Polars tutorial for lazy execution (#40).

1. it changes and add sentences to resolve the ambiguous meaning of the sentence "Unless specified, Polars defaults to the pl.String datatype for all data.",
2. deletes an empty markdown cell.

📋 Checklist

• I have included package dependencies in the notebook file using --sandbox
• If adding a course, include a README.md
• Keep language direct and simple.

[Haleshot]

Thanks for the correction 🚀