Fix dead links http://list.zfsonlinux.org

Originally, I wanted to point to directly to
https://zfsonlinux.topicbox.com/groups/zfs-discuss
as the text refers to that specific mailing list, but George Melikov
requested to change it to the general to give users the overview.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Marcel Schilling <marcel.schilling@uni-luebeck.de>
Closes #10367
Closes #10369