input: Write a python function to multiply each element in a list by two.
  assert multiply_by_two([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert multiply_by_two([6, 7, 8, 9, 10]) == [12, 14, 16, 18, 20]
  assert multiply_by_two([0, -1, -2, -3, -4]) == [0, -2, -4, -6, -8]
output: def multiply_by_two(lst):
      return [x * 2 for x in lst]