fixed quicksort partition and secondsmallest function

[qyynuaa]

1. https://donsheehy.github.io/datastructures/fullbook.html#quicksort
   The version without all comments looks like has some problem in last two lines
   #Put the pivot in place.
   L[pivot], L[i] = L[i], L[pivot]
   return i
   I think we still need to compare L[pivot] with L[i].
   An example case is [1,2]. If we directly swap them, the result will be [2,1].

2. https://donsheehy.github.io/datastructures/fullbook.html#selection
   It looks like there is a typo in the secondsmallest function
   def secondsmallest(L):
   a, b = None, None
   for item in L:
   if a is None or item <= b: # it should be item <=a I guess
   a, b = item, a
   elif b is None or item <= a: # it should be item <= b I guess
   b = item
   return b

3. https://donsheehy.github.io/datastructures/fullbook.html#the-quickselect-algorithm
   The quick selection uses the partition function. I think we still need to compare compare L[pivot] with L[i] to decide whether to swap them or not.

[ziqiyang107]

quick selection code doesn't work sometimes, need to fix