Pin dependency ex_doc to 0.40.1 (#418)

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

Author: renovate[bot]

mix.exs

@@ -51,7 +51,7 @@ defmodule EctoNestedChangeset.MixProject do
       {:dialyxir, "1.4.7", only: [:dev], runtime: false},
       {:ecto, "~> 3.7"},
       {:ecto_sql, "3.13.4", only: :test},
-      {:ex_doc, "== 0.40.1", only: :dev, runtime: false},
+      {:ex_doc, "0.40.1", only: :dev, runtime: false},
       {:excoveralls, "0.18.5", only: :test},
       {:stream_data, "1.2.0", only: [:dev, :test]}
     ]