docs: fix incorrect example outputs in addition_subtraction.md

The hours and minutes example outputs in the addition/subtraction
documentation contained three incorrect values:
- add(hours=1) after add(hours=24): '2012-02-25' → '2012-01-29'
- subtract(hours=1): '2012-02-29' → '2012-01-29' (also Feb 29 doesn't
  exist in 2012, it's a leap year but still wrong context)
- subtract(minutes=24) after starting at 01:01: '00:00:00' → '00:37:00'

Fixes #925

Author: frankgoldfish

docs/docs/addition_subtraction.md

@@ -51,9 +51,9 @@ Each method returns a new `DateTime` instance.
 >>> dt = dt.add(hours=24)
 '2012-01-29 00:00:00'
 >>> dt = dt.add(hours=1)
-'2012-02-25 01:00:00'
+'2012-01-29 01:00:00'
 >>> dt = dt.subtract(hours=1)
-'2012-02-29 00:00:00'
+'2012-01-29 00:00:00'
 >>> dt = dt.subtract(hours=24)
 '2012-01-28 00:00:00'
 
@@ -64,7 +64,7 @@ Each method returns a new `DateTime` instance.
 >>> dt = dt.subtract(minutes=1)
 '2012-01-28 01:01:00'
 >>> dt = dt.subtract(minutes=24)
-'2012-01-28 00:00:00'
+'2012-01-28 00:37:00'
 
 >>> dt = dt.add(seconds=61)
 '2012-01-28 00:01:01'