get mix docs working

Author: mweidner037

mix.exs

@@ -30,7 +30,7 @@ defmodule Mongo.Ecto.Mixfile do
       {:credo, "~> 1.5.6", only: [:dev, :test], runtime: false},
       {:dialyxir, "~> 1.1.0", only: :dev, runtime: false},
       {:ecto, "~> 3.12"},
-      {:ex_doc, ">= 0.0.0", only: :dev, runtime: false},
+      {:ex_doc, ">= 0.0.0", only: :docs, runtime: false},
       {:excoveralls, "~> 0.16", only: :test},
       {:mongodb_driver, "~> 1.4"},
       {:telemetry, ">= 0.4.0"}