Use groonga-talk instead of groonga-users-en for English ML

I think that groonga-talk ML is used, but groonga-users-en ML isn't used.

  * groonga-talk
    * http://lists.sourceforge.net/mailman/listinfo/groonga-talk
  * groonga-users-en
    * http://rubyforge.org/mailman/listinfo/groonga-users-en