"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

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

No worries if anything in these lists is unclear. Just submit the PR and ask away! :+1:

---

Things to check before submitting a PR

• [ ] the tests are passing locally with cargo test
• [ ] cookbook renders correctly in mdbook serve -o
• [x] commits are squashed into one and rebased to latest master
• [x] 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

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! :smile: