input: Write a python function to find the index of the maximum number in a list.
  assert find_max_index([5, 10, 20, 6, 8])==2
  assert find_max_index([1, 3, 7, 2, 5])==2
  assert find_max_index([10, 15, 5, 30, 20])==3
output: def find_max_index(lst):
      return lst.index(max(lst))