Fix percent-encoding link

[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
[ ] 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

rust-lang-nursery / rust-cookbook