chore(deps): update dependency ex_doc to ~> 0.39 (#925)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

apps/opentelemetry_api_experimental/mix.exs

@@ -44,7 +44,7 @@ defmodule OpenTelemetryExperimental.MixProject do
     end)
     |> Enum.concat([
       {:cmark, "~> 0.10", only: :dev, runtime: false},
-      {:ex_doc, "~> 0.38", only: :dev, runtime: false},
+      {:ex_doc, "~> 0.39", only: :dev, runtime: false},
       {:dialyxir, "~> 1.0", only: [:dev], runtime: false},
       {:covertool, ">= 0.0.0", only: :test}
     ])