Update the documentation for the initializer

[johan-smits]

The current default value for the initializer config is:

AjaxDatatablesRails.configure do |config|
  # available options for db_adapter are: :oracle, :pg, :mysql2, :sqlite3
  # config.db_adapter = :pg

  # available options for paginator are: :simple_paginator, :kaminari, :will_paginate
  # config.paginator = :simple_paginator
end

Suggest to make it like this because it will use always the right connector

AjaxDatatablesRails.configure do |config|
  # available options for db_adapter are: :oracle, :pg, :mysql2, :sqlite3
  # config.db_adapter = :pg

  # Or you can use your rails environment adapter if you want a generic dev and production
  config.db_adapter = Rails.configuration.database_configuration[Rails.env]['adapter'].to_sym

  # available options for paginator are: :simple_paginator, :kaminari, :will_paginate
  # config.paginator = :simple_paginator
end