input: Write a python function to format a date string from "dd-mm-yyyy" to "yyyy-mm-dd".
  assert format_date("10-05-2020") == "2020-05-10"
  assert format_date("25-12-2021") == "2021-12-25"
  assert format_date("01-01-1990") == "1990-01-01"
output: def format_date(date_str):
      day, month, year = date_str.split('-')
      return f"{year}-{month}-{day}"