input: Write a python function to find the last element of a given list.
  assert last_element([1, 2, 3, 4, 5]) == 5
  assert last_element([10, 20, 30, 40, 50]) == 50
  assert last_element(['a', 'b', 'c', 'd']) == 'd'
output: def last_element(lst):
      return lst[-1]