input: Write a function to find the index of the maximum value in a list.
  assert max_index([1, 3, 2, 8, 5, 7]) == 3
  assert max_index([10, 20, 30, 40, 50]) == 4
  assert max_index([-10, -20, -30, -5, -50]) == 3
output: def max_index(lst):
      return lst.index(max(lst))