Fix README elasticsearch-model

[katsuhisa91]

I found an inconsistency method name, inside README of elasticsearch-model's Asynchronous Callbacks section.

You wrote Article Model file as below.

class Article
  include Elasticsearch::Model

  after_save    { Indexer.perform_async(:index,  self.id) }
  after_destroy { Indexer.perform_async(:delete, self.id) }
end

But, you wrote example implementation of the Indexer worker class as below.

class Indexer
  include Sidekiq::Worker
  sidekiq_options queue: 'elasticsearch', retry: false
  ［...]
  def perform(operation, record_id)
    ［...]
  end
end

I think we should fix Indexer worker class's method name from perform to perform_async.

[estolfo]

@katsuhisa91 thanks!