op-mode: fix "show arp interface" argument level
op-mode: fix "show arp interface" argument level

Commit 84ce69a4 ('op-mode: add "show arp" command') copied the syntax from
"show protocols static arp" to "show arp" but the CLI variable reference index
was not decreases from 6 to 4 as the CLI level changed for the new command.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x