Typo
http://pandas.pydata.org/pandas-docs/dev/timeseries.html#time-span-representation

```
In [1162]: Period('2012', freq='A-DEC') - Period(2002', freq='A-DEC')
  File "<ipython-input-1162-41a08553f136>", line 1
    Period('2012', freq='A-DEC') - Period(2002', freq='A-DEC')
                                                  ^
SyntaxError: invalid syntax
```
