Update principal_component_analysis.py

[ ] 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

rasbt / mlxtend