bandwidth-file-spec.txt: bandwidth-avg is not calculated from any bandwidth burst option

    bandwidth-avg is the minimum of MaxAdvertisedBandwidth,
    BandwidthRate, RelayBandwidthRate, BandwidthBurst, and
    RelayBandwidthBurst.

(https://gitweb.torproject.org/torspec.git/tree/bandwidth-file-spec.txt#n315)

This should be:

    bandwidth-avg is the minimum of MaxAdvertisedBandwidth, BandwidthRate, RelayBandwidthRate,
    bandwidth-burst is the minimum of BandwidthBurst, and RelayBandwidthBurst.