Fix wrong link in ja/users/index.html

https://github.com/groonga/groonga.github.com ->
https://github.com/groonga/groonga.org
                                   ^^^
% gem install inplace
% inplace "sed -e 's/groonga\.github\.com/groonga.org/g'" ja/users/index.html
Merge pull request #4 from myokoym/fix-wrong-link

Fix wrong link in ja/users/index.html

Patch by Masafumi Yokoyama. Thanks!!!