Fix Broken Links

[Arteiii]

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Tasks for Fixing Broken Links

CONTRIBUTING.md

• [x] Fix or replace the link: [Normal distribution documentation](https://docs.rs/rand/*/rand/distributions/struct.Normal.html) (Not Found)

src/encoding/string/url-encode.md

• [x] Fix or replace the link: [form_urlencoded parse function](https://docs.rs/url/*/url/form_urlencoded/fn.parse.html) (Not Found)
• [x] Fix or replace the link: [form_urlencoded byte_serialize function](https://docs.rs/url/*/url/form_urlencoded/fn.byte_serialize.html) (Not Found)

src/cryptography/hashing/hmac.md

• [x] Fix or replace the link: [HMAC Signature struct](https://briansmith.org/rustdoc/ring/hmac/struct.Signature.html) (Not Found)

README.md

• [x] Fix or replace the link: [Rust Cookbook Travis CI](https://travis-ci.com/rust-lang-nursery/rust-cookbook) (Not Found)

src/development_tools/debugging/config_log/log-custom.md

• [x] Fix or replace the link: [log4rs Config struct](https://docs.rs/log4rs/*/log4rs/config/struct.Config.html) (Not Found)

src/web/scraping/broken.md

• [x] Fix or replace the link: [tokio spawn function](https://docs.rs/tokio/*/tokio/fn.spawn.html) (Not Found)

src/web/clients/api/rate-limited.md

• [x] Fix or replace the link: [Defining custom headers in hyper](https://doc.servo.org/hyper/header/index.html#defining-custom-headers) (Not Found)