Fix broken reference to app.config in "Creating our own Mix task" docs

[jyc]

To use an application's infrastructure, the docs say @requirements ["app.config"] should work, but it doesn’t:

→ mix task.name
Compiling 1 file (.ex)
** (RuntimeError) could not lookup Ecto repo App.Repo because it was not started or it does not exist
    (ecto 3.11.1) lib/ecto/repo/registry.ex:22: Ecto.Repo.Registry.lookup/1
    (ecto 3.11.1) lib/ecto/repo/supervisor.ex:160: Ecto.Repo.Supervisor.tuplet/2
    (app 0.1.0) lib/app/repo.ex:2: App.Repo.all/2
    (app 0.1.0) lib/mix/tasks/task.name.ex:13: Mix.Tasks.App.Ingest.run/1
    (mix 1.16.0) lib/mix/task.ex:478: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.16.0) lib/mix/cli.ex:96: Mix.CLI.run_task/2
    /Users/jyc/.asdf/installs/elixir/1.16.0-otp-26/bin/mix:2: (file)

I asked on the forums and it turns out that app.start is the correct name: https://elixirforum.com/t/accessing-an-ecto-repo-inside-of-a-mix-task-in-phoenix/60942/2

[chrismccord]

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