input: Write a python function to iterate over a list and return a list of its elements multiplied by 2.
  assert multiply_by_2([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert multiply_by_2([10, 20, 30]) == [20, 40, 60]
  assert multiply_by_2([5, 7, 9, 8, 6, 4, 3]) == [10, 14, 18, 16, 12, 8, 6]
output: def multiply_by_2(lst):
      return [x * 2 for x in lst]