input: Write a python function to modify a list based on a given argument, either multiplying or adding the elements to themselves.
  assert modify_list([1, 2, 3], "multiply") == [1, 4, 9]
  assert modify_list([1, 2, 3], "add") == [2, 4, 6]
  assert modify_list([5, 10], "multiply") == [25, 100]
  assert modify_list([5, 10], "add") == [10, 20]
output: def modify_list(lst, operation):
      if operation == "multiply":
          return [x * x for x in lst]
      elif operation == "add":
          return [x + x for x in lst]