The problems addressed in the earlier version led to an "incorrect"
example which is bad to pass along after reading a whole book. after
getting some feedback in #2 i decided to show how we can create a
proper `Parker`.
The main example (which I assume most interested readers will copy) now
uses a proper parking thechnique so there should be no more dataraces
left.
I also removed the "Reader Excercise" paragraph suggesting that they
explore a way to implement proper parking since we now show that in
our main example.
