"expand" -> "show hidden lines"

[noamraph]

Change the name of the button and its icon according to what's actually displayed on https://rust-lang-nursery.github.io/rust-cookbook/about.html

fixes #658

🎉 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! 👍

---

Things to check before submitting a PR

• the tests are passing locally with cargo test
• cookbook renders correctly in mdbook serve -o
• 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
• spell check runs without errors ./ci/spellcheck.sh
• link check runs without errors link-checker ./book
• 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! 😄