network: fix DHCPv6 Prefix Delegation example after option rename
    
    99e015e28c8322bf714f45cd1defcf20ac2103c5 missed to update the example
    below - DHCPv6.AssignAcquiredDelegatedPrefixAddress was moved to
    DHCPv6PrefixDelegation.Assign.
    
    As it already defaulted to true since it's introduction in
    9efa8a3cff9948d3a78597b74dca75c805716de4, there's no need to explicitly
    list it at all.