fix #644

[barrowsys]

fixes #644

love the PR template btw

---

• [x] the tests are passing locally with cargo test
• [x] cookbook renders correctly in mdbook serve -o
• [x] commits are squashed into one and rebased to latest master
  • merge is handled by the PR, right?
• [x] PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
• [x] spell check runs without errors ./ci/spellcheck.sh
• [ ] link check runs without errors link-checker ./book
  • not sure where to get link-checker?
• [x] non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• [x] links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• [x] example has standard error handling
• [x] code identifiers in description are in hyperlinked backticks

[barrowsys]

CI confirms the relevant tests worked