Fix typo in documentation of mcnemar_table

[ftnext]

Code of Conduct

Description

Thank you so much for this awesome library. I noticed a typo and send this pull request.

Related issues or pull requests

None

Pull Request Checklist

• [ ] Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• [ ] Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• [x] Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• [ ] Ran PYTHONPATH='.' pytest ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• [ ] Checked for style issues by running flake8 ./mlxtend

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[rasbt]

This looks good to me as well!