Docs: Improve custom body reader example in `Plug.Parsers` module

[tonydangblog]

Change example to use with

Change from:

{:ok, body, conn} = Plug.Conn.read_body(conn, opts)

To:

with {:ok, body, conn} <- Plug.Conn.read_body(conn, opts) do

This ensures that if Plug.Conn.read_body/2 fails, the function will return a meaningful error tuple instead of raising a MatchError. I saw this also suggested in this Dashbit article.

Remove extra parentheses

Change from:

conn = update_in(conn.assigns[:raw_body], &[body | (&1 || [])])

To:

conn = update_in(conn.assigns[:raw_body], &[body | &1 || []])

This just removes the extra parens that the Elixir formatter would remove. :)

[josevalim]

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