authentication key to text method miss-spelled

It is AuthKey::ToText() when it should be Authkey::toText().

BTW it is the occasion to fix a second issue about this method: display the key in hexadecimal (it could make harder to input it but the syntax for DHCPv6 host reservations nor in the host commands hook (can be the subject of another issue but currently authentication keys are stored only in databases, not config files).