DOC: Fix df.resample docstring example
 - [x] passes ``git diff upstream/master | flake8 --diff``

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15004?src=pr) is 84.78% (diff: 100%)
> Merging [#15004](https://codecov.io/gh/pandas-dev/pandas/pull/15004?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #15004   diff @@
==========================================
  Files           145        145          
  Lines         51090      51090          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43315      43315          
  Misses         7775       7775          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3e3434b...1df8f80](https://codecov.io/gh/pandas-dev/pandas/compare/3e3434b21e3c135c3bd7649689d488380ea1ebc1...1df8f80393eb81ad6e025a6ae777f57b8c675ce5?src=pr) @petehuang Thanks!