input: Write a python function to find the maximum number in a list.
  assert find_max([7, 12, 5, 9]) == 12
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([-10, -20, -30, -5]) == -5
output: def find_max(lst):
      return max(lst)