input: Write a python function to find the maximum value in a list of integers.
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([10, 20, 5, 7, 8]) == 20
  assert find_max([-1, -2, -3, -4]) == -1
output: def find_max(lst):
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value