Restore missing link to Ruby Central in the "Ruby Conferences"

Author: ablzh

en/community/conferences/index.md

@@ -40,5 +40,6 @@ You can also find the GitHub repository link there to add or update information
 
 [rc]: https://www.rubyevents.org/
 [1]: http://rubyconf.org/
+[2]: http://rubycentral.org/
 [3]: http://rubykaigi.org/
 [4]: http://euruko.org