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