Fix error handling link in about page

[TheJanzap]

The error handling link to the Rust Book on the About page was broken. I replaced it with the one I assumed was correct.

🎉 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]

thank you! I've got the error story rewritten in #730 but I'm still struggling with skeptic rlibs and getting rand to be testable.