Slight fixing up of the Prometheus exporter section of the Timeseries docs

[rk295]

Few nitpick things:

• The variable to enable Prometheus didn't match the code
• The default port differed from the code
• The docs suggest you can specify a path, but the code actually serves on any path

Note: I also raised https://github.com/sdr-enthusiasts/docker-adsb-ultrafeeder/pull/64 to allow changing of the port.