Modified hw2-4 and hw4-3

Prof Caffo,
I am sending this pull request in reference to this thread: https://class.coursera.org/statinference-003/forum/thread?thread_id=76. I have reproduced the thread below. 

Brief summary: I have made changes to the hw2.Rmd and hw4.Rmd files, to account for the two changes I had suggested. This changes also caused changes in the hw2.md, hw2.html, hw4.md and hw4.html files. This pull request thus encompass the following files: hw2.Rmd, hw2.md, hw2.html, hw4.Rmd, hw4.md, hw4.html.


The coursera post that I referred to above;

Here are two fixes needed to the homeworks.

HW4 Q3 asks for "...would reject the null hypothesis for...". The hint for the question suggests that it should be "... would fail to reject the null hypothesis for...". Again, the "Show Answer" button shows both lower and upper levels as 2.78 and the grader also expects 2.78 for both these levels. So this also needs a fix.

The hint for HW2 Q4 should probably be something like: x0.95, so that F(x0.95)=0.95 ( I hope I did this right?). Right now, the hint looks like a copy-paste of Q2.