Replace deprecated around_filter with around_action in README

[jimmyasyraf]

Documentation-only change: updates the memoized cache proxy example in the README to use around_action instead of around_filter.

Why

The *_filter callback names were deprecated in Rails 5.0 and removed entirely in Rails 5.1 (2017). Copying the current README example into any modern Rails app raises:

NoMethodError: undefined method 'around_filter' for ApplicationController

around_action is the direct replacement (it was introduced as an alias in Rails 4.0 and has been the canonical name since), so the example now works on all Rails versions this gem supports.

class ApplicationController < ActionController::Base
  around_action :identity_cache_memoization

  def identity_cache_memoization(&block)
    IdentityCache.cache.with_memoization(&block)
  end
end

No behavior change — with_memoization itself is untouched.

[jimmyasyraf]

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