Docs: Update example 10 with custom Iris class names (#737)

[N-Harsha-Vardhan-Dev]

Code of Conduct

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Description

This PR updates Example 10 in the plot_decision_regions user guide to demonstrate how to provide custom class names (e.g., actual species names like 'Setosa') for the legend instead of generic placeholders.

• Updated plot_decision_regions.ipynb with descriptive species labels.
• Updated the corresponding static image file in the plot_decision_regions_files directory to reflect these changes.

Related issues or pull requests

Fixes #737

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

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[rasbt]

LGTM