DOC: Typing error in ExcelFile class example
 - [x] closes #15037
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

- Tests with local build of documentation passed
- Flake8 seems to be relevant for source code changes, only
- Thought that a whatsnew entry is not relevant in this case