input: Write a python function to sort a list of tuples based on the second element of each tuple in ascending order.
  assert sort_tuples([(4, 2), (3, 7), (2, 3)]) == [(4, 2), (2, 3), (3, 7)]
  assert sort_tuples([(1, 3), (4, 2), (5, 1)]) == [(5, 1), (4, 2), (1, 3)]
  assert sort_tuples([(4, 5), (3, 4), (2, 3)]) == [(2, 3), (3, 4), (4, 5)]
output: def sort_tuples(lst):
      return sorted(lst, key=lambda x: x[1])