one-word manpage correction: snapshot->rollback (#16294)

This commit fixes what is probably a copy-paste mistake. The
dracut.zfs manpage claims that the bootfs.rollback option executes
zfs snapshot -Rf. zfs snapshot does not have a -R option. `zfs
rollback` does.

Signed-off-by: Alphan Yılmaz <alphanyilmaz@gmail.com>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>