input: Write a python function to insert a comma at a given index in a string.
  assert split_string("Hello World", 6) == "Hello,World"
  assert split_string("abcdefg", 3) == "abc,efg"
  assert split_string("1234567890", 5) == "12345,7890"
output: def split_string(s, index):
      return s[:index] + ',' + s[index:]