DOC: correct merge_asof example
- [ ] closes #19489 
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

- update of incorrect documentation example for merge_asof.
- add a test corresponding to this example.
Hello @ZhuBaohe! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 18, 2018 at 17:46 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@c4770c7`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19737/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19737   +/-   ##
=========================================
  Coverage          ?   91.58%           
=========================================
  Files             ?      150           
  Lines             ?    48862           
  Branches          ?        0           
=========================================
  Hits              ?    44750           
  Misses            ?     4112           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.95% <ø> (?)` | |
| #single | `41.75% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/19737/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.25% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=footer). Last update [c4770c7...16d4260](https://codecov.io/gh/pandas-dev/pandas/pull/19737?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!