Updated hw3.Rmd to fix Q2(hint and answer) and  Q3(typo)

Prof Caffo,
This pull request is for hw3.Rmd file. I am only submitting the .Rmd file (and not the hw3.md and hw3.html files).

I have fixed a typo in Q3. The word naturally was actually spelt as natually, in the answer options.

I have made changes to Q2 (the hint is not formatting correctly and the answer was wrong). 
Q7 also needs a change: it needs to provide an input field. But I have not made that change, since I cannot verify it.

You had actually done these changes earlier (See thread: https://class.coursera.org/statinference-003/forum/thread?thread_id=55). Somehow, both these changes got over-written.

- manohar