Fix indent and rendering across 4 different examples

[jungseoklee]

This patch fixes indent and rendering across 4 different examples:

• https://rust-lang-nursery.github.io/rust-cookbook/database/postgres.html#aggregate-data
• https://rust-lang-nursery.github.io/rust-cookbook/development_tools/versioning.html#check-if-given-version-is-pre-release
• https://rust-lang-nursery.github.io/rust-cookbook/encoding/csv.html#serialize-records-to-csv-using-serde
• https://rust-lang-nursery.github.io/rust-cookbook/file/dir.html#recursively-find-duplicate-file-names

🎉 Hi and welcome! Please read the text below and remove it - Thank you! 🎉

No worries if anything in these lists is unclear. Just submit the PR and ask away! 👍

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Things to check before submitting a PR

• the tests are passing locally with cargo xtask test all
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
  • if issue does not exist consider creating it or remove the clause
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

Things to do after submitting PR

• check if CI is happy with your PR

Thank you for reading, you may now delete this text! Thank you! 😄

[AndyGauge]

Thanks for contributing

[jungseoklee]

My pleasure. This book is really helpful.