DOC: update the to_excel docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [ x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x ] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x ] It has been proofread on language by another sprint participant

```
Errors found:
	Missing description for See Also "pd.read_excel" reference
```