gh-38314: fix typos and code details in Hecke triangle groups
This is fixing a few typos and minor code details, mostly in
`hecke_triangle_groups`
### :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/38314
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
