input: Write a python function to append 10 to each number in a list.
  assert append_nums([1, 2, 3]) == [11, 12, 13]
  assert append_nums([10, 20, 30]) == [20, 30, 40]
  assert append_nums([0, -10, -20]) == [10, 0, -10]
output: def append_nums(lst):
      return [x + 10 for x in lst]