input: Write a python function to find the maximum number in a list.
  assert find_max([2, 3, 4, 9, 5]) == 9
  assert find_max([-1, -2, -3, -4]) == -1
  assert find_max([10, 20, 30, 5, 1]) == 30
output: def find_max(lst):
      return max(lst)