Update copyright year (2019)

- Fixes #3

Author: tatsuya6502

index.md

@@ -94,7 +94,7 @@ Rustは速度、安全性、平行性の3つのゴールにフォーカスした
 
 * * *
 
-Copyright © 2018 The Rust Document Writers and Translators. Licensed under
+Copyright © 2018 – 2019 The Rust Document Writers and Translators. Licensed under
 the [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0) or
 the [MIT license](https://opensource.org/licenses/MIT), at your option.