DOC: fix typo in merge_asof docstring examples
Just a trivial fix to docs - incorrect function name in doctests.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14718?src=pr) is 85.21% (diff: 100%)
> Merging [#14718](https://codecov.io/gh/pandas-dev/pandas/pull/14718?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14718   diff @@
==========================================
  Files           143        143          
  Lines         50796      50796          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43285      43285          
  Misses         7511       7511          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [880de30...675c4b5](https://codecov.io/gh/pandas-dev/pandas/compare/880de3054c7ce3a6c48e295837207787dc73350f...675c4b537098188c6260d2108e280c12c0c9978a?src=pr) @dwillmer Thanks! (leftover from a previous iteration of the PR before the name change :-))