input: Write a function to double the elements of a list.
  assert my_list([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert my_list([10, 20, 30]) == [20, 40, 60]
  assert my_list([-1, -2, -3]) == [-2, -4, -6]
output: def my_list(lst):
      return [x * 2 for x in lst]