contrib: dracut: zfs-{rollback,snapshot}-bootfs: explicit snapname fix

Due to a missing semicolon on the ExecStart line, it wasn't possible
to specify the snapshot name on the bootfs.{rollback,snapshot}
kernel parameters if the boot dataset name was obtained from the
root=zfs:... kernel parameter.

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregory Bartholomew <gregory.lee.bartholomew@gmail.com>
Closes #13585