A couple of minor typo's in the ARM

There are a couple of typo's in examples in the ARM

doc/sphinx/arm/dhcp4-srv.rst: "Dhcp4:" {
doc/sphinx/arm/dhcp6-srv.rst: "Dhcp6:" {
doc/sphinx/arm/hooks-ddns-tuning.rst: "ddns-tuning:" {
doc/sphinx/arm/hooks.rst: "ddns-tuning:" {

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