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.

See 8db41ff7 why

I believe this paragraph should move to the end as it's a little out of the scope of Rails guides.