input: Write a function to sort a list of tuples by their second item.
  assert sort_by_second_item([('Apple', 2), ('Orange', 1), ('Banana', 3), ('Grape', 4)])==[('Orange', 1), ('Apple', 2), ('Banana', 3), ('Grape', 4)]
  assert sort_by_second_item([('a', 3), ('b', 2), ('c', 1)])==[('c', 1), ('b', 2), ('a', 3)]
  assert sort_by_second_item([(1, 10), (2, 5), (3, 8)])==[(2, 5), (3, 8), (1, 10)]
output: def sort_by_second_item(lst):
      return sorted(lst, key=lambda x: x[1])