message: Fix broken link of rust-by-example
diff: <li class="pure-menu-item"><a href="https://www.rust-lang.org/" target="_blank" class="pure-menu-link">The Rust Programming Language</a></li>
  <li class="pure-menu-item"><a href="https://doc.rust-lang.org/book/" target="_blank" class="pure-menu-link">The Book</a></li>
  <li class="pure-menu-item"><a href="https://doc.rust-lang.org/std/" target="_blank" class="pure-menu-link">Standard Library API Reference</a></li>
  - <li class="pure-menu-item menu-item-divided"><a href="http://rustbyexample.com/" target="_blank" class="pure-menu-link">Rust by Example</a></li>
  + <li class="pure-menu-item menu-item-divided"><a href="https://doc.rust-lang.org/rust-by-example/" target="_blank" class="pure-menu-link">Rust by Example</a></li>
  <li class="pure-menu-item"><a href="https://crates.io" target="_blank" class="pure-menu-link">Crates.io</a></li>
  <li class="pure-menu-item menu-item-divided"><a href="http://doc.crates.io/guide.html" target="_blank" class="pure-menu-link">The Cargo Guide</a></li>
  