fpinscala-parallelism: Explains that the example map2 is semantically incorrect

Problem
======
The function returned by the example map2 waits for the underlying Futures to be finished before returning.  This means that
```scala
exampleMap2(fork(par1), fork(par2), fn)
```
Will block and wait for fork to be finished for both of them.

Solution
======
Clarify the comment.  Providing a correct implementation is tantamount to solving the exercise, which would be a waste.

Extra
====
Fixed a memory visibility bug.