input: Write a python function to swap two elements in a list at given indices.
  assert swap([10, 20, 30, 40, 50], 3, 4) == [10, 20, 30, 50, 40]
  assert swap([1, 2, 3, 4, 5], 0, 2) == [3, 2, 1, 4, 5]
  assert swap([9, 8, 7, 6], 1, 3) == [9, 6, 7, 8]
output: def swap(lst, index1, index2):
      lst[index1], lst[index2] = lst[index2], lst[index1]
      return lst