Add Error to Result added note for tokio

[sain-VO]

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Added error to Result for main, otherwise it won't compile. If tokio isn't added with features = ["full"] main won't work, so I thought it would be a good idea to remind them.

fixes #ISSUE_ID

: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:

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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! :smile: