Update default.conf to avoid peaks in network graph

On every range overflow of the 32bit network counter there were peaks in network graph for the up and download speed. Setting the RRD to COUNTER fixes this.
It's necessary to delete the RRDs for "net_send" and "net_received" to renew the RRD-Databases during next restart of RPi-Monitor.
For more description see: https://github.com/XavierBerger/RPi-Monitor/issues/19#issuecomment-25920937