[Cs3] typos in example quiz

[Mikhail Nesterenko]

CS3 students,

The example quiz has typos.

* The correct answer to question number 4 is "b", not "c". I updated the answer key. 
* question number 7 is ambiguous since the order of function execution
  in the stacked extraction operator is undefined. The question has
  since been corrected and/or replaced. I made a note of that in the
  text of example quiz.

Thanks,
-- 
Mikhail