Possible errors in Chapter 15

[rodrigocarlson]

1. "Now that we have Time.from_seconds," should read "Now that we have Time.int_to_time,".
2. "print_time(end)" should read "end.print_time()".
3. "assert other.is_valid(), 'self is not a valid Time'" should read "assert other.is_valid(), 'other is not a valid Time'".