Add escape characters in percent-encoding example

Hi!

I found the characters + and % are alse should be escaped when followed the section about percent-encoding. So I try to fix it.

[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
[x] spell check runs without errors ./ci/spellcheck.sh
[x] link check runs without errors link-checker ./book
[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

rust-lang-nursery / rust-cookbook