Fix wrongly indented nested list in compiler.rst.

[ezio-melotti]

While working on a new sphinx-lint checker, I found a hard-to-notice indentation error, that created a definition list with the indented sublist in the <dt>. The only visibile effect was some extra indentation in the sublist:
Before:

After:

---

📚 Documentation preview 📚: https://cpython-devguide--1183.org.readthedocs.build/