Fix Japanese text hiragana -> kanji (#1812)

README.md

@@ -116,7 +116,7 @@ All documentation is available on the Gin website.
 - [English](https://gin-gonic.com/docs/)
 - [简体中文](https://gin-gonic.com/zh-cn/docs/)
 - [繁體中文](https://gin-gonic.com/zh-tw/docs/)
-- [にほんご](https://gin-gonic.com/ja/docs/)
+- [日本語](https://gin-gonic.com/ja/docs/)
 
 ## Examples