Fix a typo in "Shifting time to transition" example

[baryluk]

>>> import pendulum
>>> dt = pendulum.datetime(2013, 3, 31, 1, 59, 59, 999999, tz='Europe/Paris')
>>> print(dt)
2013-03-31T01:59:59.999999+01:00
>>> dt = dt.add(microseconds=1)
>>> print(dt)
2013-03-31T03:00:00+02:00
>>> dt.subtract(microseconds=1)
DateTime(2013, 3, 31, 1, 59, 59, 999999, tzinfo=Timezone('Europe/Paris'))
>>> print(dt.subtract(microseconds=1))
2013-03-31T01:59:59.999999+01:00
>>> 

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.