Fix incorrect subtract result in docs

The result of dt.subtract(minutes=24) from 01:01:00 should be 00:37:00, not 00:00:00.

Fixes #925

Author: daniel7an

docs/docs/addition_subtraction.md

@@ -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'