sysutils/dirvish: Fix typo in pkg-descr

Pull Request:	https://github.com/freebsd/freebsd-ports/pull/127

sysutils/dirvish/pkg-descr

-This is a network-based backup system based of rync.  It can do full and
+This is a network-based backup system based of rsync.  It can do full and
 incremental backups to a remote file server.  It relies on rsync for the
 host to host copy and uses hard links to provide multiple snapshots of the
 data without duplicating identical files.