DOC: Clean up CSV sniffing and chunking examples
Sniffing:
- Had an unused "to_csv" call

Chunking:
- Used an unnecessary "sep"
- Had two no-op statements, which seem like they were meant to be printed

Both
- Put the index as a column unnecessarily

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- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
thx @wjandrea 