DOC: Default engine incorrect for ExcelWriter and xlsx
### 

- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.ExcelWriter.html#pandas.ExcelWriter

### Documentation problem

Unless, I'm missing something I don't think openpyxl is the default for writing xlsx files.

```python
In [1014]: pd.ExcelWriter("foo.xlsx").engine
Out[1014]: 'xlsxwriter'

In [1015]: pd.ExcelWriter("foo.xlsx", engine="openpyxl").engine
Out[1015]: 'openpyxl'
``` 

### Suggested fix for documentation

Change the default to xlsxwriter in the docstring. Happy to submit a PR, if I'm correct.