fix bug fetching a range starting on 777
fix bug fetching a range starting on 777

A typo in the service name used to construct the virtual register
777 (that computes from two other real dbus paths) caused the
service not to be found if the range started on 777.

victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp,victronenergy/dbus_modbustcp