Fix typo in pysa_basics.md

[sixlettervariables]

Pre-submission checklist

• [ ] I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• [ ] pre-commit run

Summary

Fixed a small typo.

Test Plan

Manual review of code and preview of markdown.

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