fixed a typo in timezones docs.

Author: mitnk

docs/topics/i18n/timezones.txt

@@ -509,7 +509,7 @@ Setup
  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)