Update sample code for on_mount authentication

[Sleepful]

This fails %{"user_id" => user_id} = _session because session does not contain user_id, however it does contain user_token.

I'm using:

  defp deps do
    [
      {:phoenix, "~> 1.6.13"},
      {:phoenix_html, "~> 3.0"},
      {:phoenix_live_reload, "~> 1.2", only: :dev},
      {:phoenix_live_view, "~> 0.18.3"},
      {:plug_cowboy, "~> 2.5"},
    ]
  end

[josevalim]

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