fixed table layout for McNemar's test

[mmarius]

Description

Fixing the table layout for McNemar's test and changed corresponding unit tests accordingly.

Related issues or pull requests

Fixes #664

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)
• 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

[coveralls]

Coverage decreased (-0.1%) to 90.66% when pulling 8c82cf6 on mmarius:issue#664 into 71942e1 on rasbt:master.