Correct python option render

[zhongjiajie]

Old content will render to -python=2.5
This patch correct it to --python=2.5

Thanks for contributing a pull request, see checklist all is good!

• wrote descriptive pull request text
• added/updated test(s): just docs changes and don't need test
• updated/extended the documentation
• added news fragment in docs/changelog folder: minor change and not nedd

[zhongjiajie]

�I'm new in virtualenv and I confuse why only docs change will cause tests/activation/test_activation.py::test_xonosh test error? Is there someone clarification

[zhongjiajie]

Thanks for merging.