Correct default value of unknown_asset_fallback

[lazyatom]

Rails.application.config.assets.unknown_asset_fallback now defaults to false in an unmodified Rails application, but the guides say it defaults to true. This commit fixes the guide.

Summary

The default value of this configuration option is false; I believe this is true as of Rails 5.1, and certainly is in Rails 5.2; this was verified by generating a new Rails application, and running the following in a development console:

Rails.application.config.assets.unknown_asset_fallback => false
Sprockets::Railtie.config.assets.unknown_asset_fallback # => false
ActionView::Base.unknown_asset_fallback # => false

The setting of this value can be seen here:

rails/railties/lib/rails/application/configuration.rb

Line 84 in 0ec23ef

assets.unknown_asset_fallback = false

[rails-bot]

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