input: Write a python function to format an address string into a specific format.
  assert format_address('60 Fifth Avenue, New York NY')=='60 Fifth Avenue, New York, NY'
  assert format_address('123 Main Street, Los Angeles CA')=='123 Main Street, Los Angeles, CA'
  assert format_address('456 Elm Street, San Francisco CA')=='456 Elm Street, San Francisco, CA'
output: def format_address(address):
      parts = address.rsplit(' ', 1)
      return f'{parts[0]}, {parts[1]}'