Fix example showing of joined fields passed into fetch_cursor_value_fun

[nickdichev]

The fetch_cursor_value function only passes cursor_field to the fetch_cursor_value_fun:

  defp fetch_cursor_value(schema, %Config{
         cursor_fields: cursor_fields,
         fetch_cursor_value_fun: fetch_cursor_value_fun
       }) do
    cursor_fields
    |> Enum.map(fn
      {cursor_field, _order} ->
        {cursor_field, fetch_cursor_value_fun.(schema, cursor_field)}

      cursor_field when is_atom(cursor_field) ->
        {cursor_field, fetch_cursor_value_fun.(schema, cursor_field)}
    end)
    |> Map.new()
    |> Cursor.encode()
  end

The documentation example is incorrect because it shows the entire {cursor_field, direction} tuple is passed as the second parameter.

[sgerrand]

Thank you @nickdichev for your contribution. Well spotted. LGTM. 👍