Use named link reference for Discord link (en)

Author: stomar

en/community/index.md

@@ -23,7 +23,7 @@ to start:
   languages. If you have questions about Ruby, asking them on a mailing
   list is a great way to get answers.
 
-[Ruby Discord Server](https://discord.gg/UyQnKrT)
+[Ruby Discord Server][ruby-discord]
 : The Ruby Language Discord Server is a place where you can
   chat with other Rubyists, get help with Ruby questions, or help others.
   Discord is a good entry point for new developers and it is easy to join.
@@ -61,5 +61,6 @@ General Ruby Information
 
 
 [ruby-central]: http://rubycentral.org/
+[ruby-discord]: https://discord.gg/UyQnKrT
 [ruby-opendir]: https://dmoztools.net/Computers/Programming/Languages/Ruby/
 [rails-opendir]: https://dmoztools.net/Computers/Programming/Languages/Ruby/Software/Frameworks/Rails/