Use the correct NTP dhcp4 option

cmd/dhcp4d/dhcp4d.go

@@ -276,7 +276,7 @@ func newSrv(permDir string) (*srv, error) {
 		dhcp4.OptionSubnetMask:                 []byte{255, 255, 255, 0},
 		dhcp4.OptionRouter:                     []byte(serverIP),
 		dhcp4.OptionDomainNameServer:           []byte(serverIP),
-		dhcp4.OptionTimeServer:       []byte(serverIP),
+		dhcp4.OptionNetworkTimeProtocolServers: []byte(serverIP),
 		dhcp4.OptionDomainName:                 []byte(*domain),
 		dhcp4.OptionDomainSearch:               domainSearch,
 	}