Take-a-number needs a named function example

[angelikatyborska]

Partially addresses https://github.com/exercism/elixir/issues/1482

In take-a-number, it was difficult to figure out how to pass a named function to spawn. The example in the introduction used an anonymous function, and we actually don't teach anywhere how to use the capture operator to pass named functions as arguments. The only place this is mentioned, is the anonymous function's concept's "about" page, which most people will never see (rendered only if you browse the concept after learning it). None of the concept introductions mention how to use the capture operator for named functions.

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[angelikatyborska]

A bit late, but I noticed now that the exemplar solution doesn't even use function capturing 🙃 I'm afraid this change might have added even more confusion...

defmodule TakeANumber do
  def start() do
    spawn(fn -> loop(0) end)
  end

  defp loop(state) do
    receive do
      {:report_state, sender_pid} ->
        send(sender_pid, state)
        loop(state)

      {:take_a_number, sender_pid} ->
        state = state + 1
        send(sender_pid, state)
        loop(state)

      :stop ->
        nil

      _ ->
        loop(state)
    end
  end
end