DOC: Complete first sentence in DataFrame.hist (#41421).
Complete sentence and mark column and figsize as optional parameters, following suggested fix.

- [ ] closes #41421
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

thanks @mdhsieh 