Change various problem texts to give greater clarity, fix typos, etc.

Just some minor changes for some of the problem texts while I was going through the problems.

Anyway, I felt that some of the later problems such as async loops and function spy could have used more guided hints to nudge people with no absolute idea towards the intended path, but at the same time, I am not too sure if that would be giving too much away instead of encouraging people to read further on. =| Let's discuss on Thursday!