Fix links to Erlang/OTP official document

[mopp]

Please review and merge https://github.com/deadtrickster/prometheus.erl/pull/141 in advance.

Overview

Some links are suffixed by edown e.g., http://erlang.org/doc/man/mnesia.md#system_info-1

We should use http://www.erlang.org/doc/man/ instead of http://erlang.org/doc/man/ because the edown recognizes the former. Otherwise it replaces .html to .md. https://github.com/uwiger/edown/blob/754be25f71a04099c83f3ffdff268e70beeb0021/src/edown_lib.erl#L44-L52