fixed a typo in timezones docs.

   Finally, our calendar system contains interesting traps for computers::

       >>> import datetime

       >>> def substract_one_year(value):       # DON'T DO THAT!

       >>> def one_year_before(value):       # DON'T DO THAT!

       ...     return value.replace(year=value.year - 1)

       >>> one_year_before(datetime.datetime(2012, 3, 1, 10, 0))

       datetime.datetime(2011, 3, 1, 10, 0)