Fix an editing fragment in the book forms chapter

I think there is an editing fragment in the Forms chapter. In the "More sophisticated fields" section, the type signature of the carAForm function changed to: 
carAForm :: Maybe Car -> AForm Synopsis Synopsis Car 
which looks wrong. I think it should be: 
carAForm :: Maybe Car -> AForm Handler Car

or if the type changed on purpose, an explanation about why it changed would be useful.