input: Write a function to sort a list of tuples according to their second parameter.
  assert sort_tuples([(1, 3), (3, 2), (2, 1)]) == [(2, 1), (3, 2), (1, 3)]
  assert sort_tuples([(4, 5), (2, 2), (1, 3)]) == [(2, 2), (1, 3), (4, 5)]
  assert sort_tuples([(5, 1), (1, 2), (3, 1)]) == [(5, 1), (3, 1), (1, 2)]
output: def sort_tuples(lst):
      return sorted(lst, key=lambda x: x[1])