README: Update OpenZFS website url

This change is to first replace the OpenZFS website in the README to
point to openzfs.org as this is what open-zfs.org redirects to.
Along with replacing the URL, the protocol is also upgraded
from http to https.

These changes should prevent web browsers such as Firefox from
complaining about visiting a http site, if the proper security
settings are enabled, when it will still end up on a https page
after the redirect.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Hirasaki <ryanhirasaki@gmail.com>
Closes #12939