modified QUESTIONS.md to ensure peterson.s is called correct

hw7/simu1/QUESTIONS.md

 Now let’s look at the code in `peterson.s`, which implements Peterson’s
 algorithm (mentioned in a sidebar in the text). Study the code and see if you
 can make sense of it.
+To run `peterson.s` correctly, the -a flag has to be set with the values `bx=0,bx=1` to ensure that the two threads have different IDs.
 
 1. Now run the code with different values of -i. What kinds of different
    behavior do you see?