DOC: fix groupby.rst building errors #14861
Rename _resample()_ `df` to `df_re` under **New syntax to window and resample operations** section.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14863?src=pr) is 85.26% (diff: 100%)
> Merging [#14863](https://codecov.io/gh/pandas-dev/pandas/pull/14863?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

```diff
@@             master     #14863   diff @@
==========================================
  Files           144        144          
  Lines         50980      50980          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43469      43469          
  Misses         7511       7511          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [0c82abe...7282490](https://codecov.io/gh/pandas-dev/pandas/compare/0c82abef8bc8fc7419a3b8823229cafd195595ac...728249016b33f753e04fbcbecda112495b33f897?src=pr) thanks!