DOC: fix RT03,SA01,ES01 for pandas.core.resample.Resampler.__iter__
fixes

```
pandas.core.resample.Resampler.__iter__ RT03,SA01,ES01
pandas.core.groupby.DataFrameGroupBy.__iter__ RT03,SA01
pandas.core.groupby.SeriesGroupBy.__iter__ RT03,SA01
```
Thanks @tuhinsharma121!