contrib: dracut: README.md

Change zfs-snapshot-bootfs.service to zfs-rollback-bootfs.service in
cmdline point 4 of README.md.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregory Bartholomew <gregory.lee.bartholomew@gmail.com>
Closes #13609