sysutils/zfs-stats: Update WWW

Since we lost sight of the fact that this port is of a fork, it is no longer
appropriate to use the original (non-fork) URL for WWW purposes.
Instead, use the URL of the fork.

PR: 258238
Approved by: zirias (mentor), mm (maintainer, timeout)
Differential Revision: https://reviews.freebsd.org/D42397