This reverts commit d8596c4b.

Reason: too many mistakes [ci skip]

This reverts commit 25648ce3.

Specifically, remove `ActionDispatch::Head` in favor of `Rack::Head`.

regarding linking to Kindle

Since ActionView::Helpers::ActiveRecordHelper is removed.

I've intentionally left out some spots as they're the expected output, and since Hash#inspect yields `=>` I guess it may be confusing for beginners to see one syntax in the guide and a different one in their system.