gh-35254: Fix docstring of `src/sage/rings/lazy_series.py`
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### 📚 Description
While browsing the documentation, I noticed some minor typesetting
issues in `src/sage/rings/lazy_series.py`. This PR aims to fix this.
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URL: https://github.com/sagemath/sage/pull/35254
Reported by: David Ayotte
Reviewer(s): David Ayotte, David Coudert, Martin Rubey
