fix homework 1 question 1, minus sign and concept

There was a missing minus sign in the explanation.  So, the question looked fine, but actually it had a flaw.  I checked the forums to be sure there was a mistake.  This forum post shows another mistake, that the explanation really fits a different question, where exactly one of the parents has the flu.
https://class.coursera.org/statinference-002/forum/thread?thread_id=180#post-946