DOC: Incorrect example result in 'merge_asof' docstring
Last example result  is incorrect for 'merge_asof' functon in [document](http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.merge_asof.html).

```python
>>> trades = pd.DataFrame({
...                'time': pd.to_datetime(['20160525 13:30:00.023',
...                                                     '20160525 13:30:00.038',
...                                                     '20160525 13:30:00.048',
...                                                     '20160525 13:30:00.048',
...                                                     '20160525 13:30:00.048']),
...                 'ticker': ['MSFT', 'MSFT', 'GOOG', 'GOOG', 'AAPL'],
...                 'price': [51.95, 51.95, 720.77, 720.92, 98.00],
...                 'quantity': [75, 155, 100, 100, 100]},
...                 columns=['time', 'ticker', 'price', 'quantity'])
>>> quotes = pd.DataFrame({
...                 'time': pd.to_datetime(['20160525 13:30:00.023',
...                                                      '20160525 13:30:00.023',
...                                                      '20160525 13:30:00.030',
...                                                      '20160525 13:30:00.041',
...                                                      '20160525 13:30:00.048',
...                                                      '20160525 13:30:00.049',
...                                                      '20160525 13:30:00.072',
...                                                     '20160525 13:30:00.075']),
...                   'ticker': ['GOOG', 'MSFT', 'MSFT', 'MSFT', 'GOOG', 'AAPL', 'GOOG', 'MSFT'],
...                   'bid': [720.50, 51.95, 51.97, 51.99, 720.50, 97.99, 720.50, 52.01],
...                   'ask': [720.93, 51.96, 51.98, 52.00, 720.93, 98.01, 720.88, 52.03]},
...                   columns=['time', 'ticker', 'bid', 'ask'])
```
The following changes should be made.

```python
>>> pd.merge_asof(trades, quotes,
...                       on='time',
...                       by='ticker',
...                       tolerance=pd.Timedelta('10ms'),
...                       allow_exact_matches=False)
                     time ticker   price  quantity    bid    ask
0 2016-05-25 13:30:00.023   MSFT   51.95        75    NaN    NaN
1 2016-05-25 13:30:00.038   MSFT   51.95       155  51.97  51.98
2 2016-05-25 13:30:00.048   GOOG  720.77       100    NaN    NaN
3 2016-05-25 13:30:00.048   GOOG  720.92       100    NaN    NaN
4 2016-05-25 13:30:00.048   AAPL   98.00       100    NaN    NaN
```
