gh-37652: fix typos and no "Algebra" in the doc
This fixes a bunch of typos in the doc, and also rewrites one example to
use `Parent` instead of the auld `Algebra`
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/37652
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
