DOC: Fixed PR08 docstring errors in pandas.DataFrame
This relates to: [27977](https://github.com/pandas-dev/pandas/issues/27977). I have fixed the doc PR08 formatting issues for:
```
pandas.DataFrame.insert: Parameter "column" description should start with a capital letter
pandas.DataFrame.apply: Parameter "raw" description should start with a capital letter
pandas.DataFrame.pipe: Parameter "func" description should start with a capital letter
pandas.DataFrame.pipe: Parameter "args" description should start with a capital letter
pandas.DataFrame.pipe: Parameter "kwargs" description should start with a capital letter
pandas.DataFrame.corr: Parameter "method" description should start with a capital letter
pandas.DataFrame.corrwith: Parameter "axis" description should start with a capital letter
pandas.DataFrame.corrwith: Parameter "method" description should start with a capital letter
pandas.DataFrame.drop_duplicates: Parameter "keep" description should start with a capital letter
pandas.DataFrame.duplicated: Parameter "keep" description should start with a capital letter
pandas.DataFrame.idxmax: Parameter "axis" description should start with a capital letter
pandas.DataFrame.idxmin: Parameter "axis" description should start with a capital letter
pandas.DataFrame.unstack: Parameter "fill_value" description should start with a capital letter
pandas.DataFrame.plot: Parameter "kind" description should start with a capital letter
pandas.DataFrame.plot: Parameter "style" description should start with a capital letter
pandas.DataFrame.to_feather: Parameter "fname" description should start with a capital letter
pandas.DataFrame.to_msgpack: Parameter "path" description should start with a capital letter
pandas.DataFrame.to_msgpack: Parameter "append" description should start with a capital letter
pandas.DataFrame.to_msgpack: Parameter "compress" description should start with a capital letter
pandas.DataFrame.to_clipboard: Parameter "excel" description should start with a capital letter
```

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Will continue to work through all PR08 docstring errors.