Update Japanese mirror to official URLs of Ring Server

Currently URLs of backend servers are listed. And some of them fail to
be connected. So replace them with official URLs of Ring Server.

Author: Yasuhiro Kimura

en/downloads/mirrors/index.md

@@ -15,21 +15,15 @@ Please try to use a mirror that is near you.
 ### Mirror sites via HTTP
 
 * [CDN][mirror-https-cdn] (fastly.com)
-* Japan (RingServer)
-  * [shibaura-it.ac.jp][mirror-http-jp-ring-shibaura-it]
-  * [u-toyama.ac.jp][mirror-http-jp-ring-u-toyama]
-  * [airnet.ne.jp][mirror-http-jp-ring-airnet]
+* [Japan][mirror-http-jp-ring] (RingServer)
 * [Holland][mirror-http-nl] (XS4ALL) - only release packages
 * [France][mirror-http-fr] (cyberbits.eu)
 * [China 2][mirror-http-cn2] (Ruby China)
 * [South Korea][mirror-http-kr] (Korea FreeBSD Users Group)
 
 ### Mirror sites via FTP
 
-* Japan (RingServer)
-  * [shibaura-it.ac.jp][mirror-ftp-jp-ring-shibaura-it]
-  * [u-toyama.ac.jp][mirror-ftp-jp-ring-u-toyama]
-  * [airnet.ne.jp][mirror-ftp-jp-ring-airnet]
+* [Japan][mirror-ftp-jp-ring] (RingServer)
 * [Japan 3][mirror-ftp-jp3] (IIJ)
 * [South Korea][mirror-ftp-kr] (Korea FreeBSD Users Group)
 * [Germany][mirror-ftp-de] (FU Berlin)
@@ -41,17 +35,12 @@ Please try to use a mirror that is near you.
 * [South Korea][mirror-rsync-kr] (Korea FreeBSD Users Group)
 
 [mirror-https-cdn]: https://cache.ruby-lang.org/pub/ruby/
-[mirror-http-jp-ring-shibaura-it]: http://ring.shibaura-it.ac.jp/archives/lang/ruby/
-[mirror-http-jp-ring-u-toyama]: http://ring.u-toyama.ac.jp/archives/lang/ruby/
-[mirror-http-jp-ring-airnet]: http://ring.airnet.ne.jp/archives/lang/ruby/
+[mirror-http-jp-ring]: http://www.ring.gr.jp/pub/lang/ruby/
 [mirror-http-nl]: http://www.xs4all.nl/~hipster/lib/mirror/ruby/
 [mirror-http-fr]: https://mirror.cyberbits.eu/ruby/
 [mirror-http-cn2]: https://cache.ruby-china.com/pub/ruby/
 [mirror-http-kr]: http://ftp.kr.freebsd.org/pub/ruby/
-[mirror-ftp-jp-ring-shibaura-it]: ftp://ring.shibaura-it.ac.jp/pub/lang/ruby/
-[mirror-ftp-jp-ring-u-toyama]: ftp://ring.u-toyama.ac.jp/pub/lang/ruby/
-[mirror-ftp-jp-ring-airnet]: ftp://ring.airnet.ne.jp/pub/lang/ruby/
-[mirror-ftp-jp-ring-maffin]: ftp://ring.maffin.ad.jp/pub/lang/ruby/
+[mirror-ftp-jp-ring]: ftp://ftp.ring.gr.jp/pub/lang/ruby/
 [mirror-ftp-jp3]: ftp://ftp.iij.ad.jp/pub/lang/ruby/
 [mirror-ftp-kr]: ftp://ftp.kr.freebsd.org/pub/ruby/
 [mirror-ftp-de]: ftp://ftp.fu-berlin.de/unix/languages/ruby/