secondsmallest() in 11_selection.md is broken

[natanlao]

For example:

>>> def secondsmallest(L):
...     a, b = None, None
...     for item in L:
...         if a is None or item <= b:
...             a, b = item, a
...         elif b is None or item <= a:
...             b = item
...     return b
>>> secondsmallest([4, 5, 1, 2, 3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in secondsmallest
TypeError: '<=' not supported between instances of 'int' and 'NoneType'

secondsmallest() does not account for when the second element in L is larger than the first element.