Update Sentry.Config moduledoc

I would suggest this edit because I think it can be an easy mistake to copy paste the tuple, change the env var and forget about the quotes (it happened to me 😅 )

Author: LysanderGG

lib/sentry/config.ex

@@ -2,7 +2,7 @@ defmodule Sentry.Config do
   @moduledoc """
   This module provides the functionality for fetching configuration settings and their defaults.
 
-  Sentry supports loading config at runtime, via `{:system, SYSTEM_ENV_KEY}` tuples, where Sentry will read `SYSTEM_ENV_KEY` to get the config value from the system environment at runtime.
+  Sentry supports loading config at runtime, via `{:system, "SYSTEM_ENV_KEY"}` tuples, where Sentry will read `SYSTEM_ENV_KEY` to get the config value from the system environment at runtime.
   """
 
   @default_included_environments [:dev, :test, :prod]