Update edown 0.8.4

[mopp]

Why

❯ erlenv release
24.2 (set by /Users/mopp/.anyenv/envs/erlenv/release)

> erl
Erlang/OTP 24 [erts-12.2] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit]

Eshell V12.2  (abort with ^G)
1>

I've used Erlang/OTP 24.2 and I found rebar3 edoc causes the error below because I tried to fix the documents. See also https://github.com/deadtrickster/prometheus.erl/pull/142

> ./_build/rebar3 edoc
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling edown
===> Compiling _build/docs/lib/edown/src/edown_doclet.erl failed
_build/docs/lib/edown/src/edown_doclet.erl:112:15: record context undefined
_build/docs/lib/edown/src/edown_doclet.erl:113:15: record context undefined
_build/docs/lib/edown/src/edown_doclet.erl:114:20: record context undefined

What I did

I just update the down version and regenerate the document.