Add comment in L02, line1386(and output fix)

[ChenglinXie]

Add notes in the section: Abbreviated Assignment 1.Add notes to better illustrate what "//" does 2.And I think the output of here is wrong. Because original x(dividend) is 9.5, x //= 2 means x = x // 2, then x = 9.5 // 2, then the output should be 4.0 not 2.