Fix Prometheus TSDB file layout explanation in comparison doc

[juliusv]

https://prometheus.io/docs/introduction/comparison/ has "Prometheus also creates one local file per time series". This is no longer true for Prometheus 2.x and needs to be updated.

[brian-brazil]

The comparison doc is out of date in a few places, this is on my todo list.

[jan--f]

Hello from the bug scrub!

This is still very relevant.