gh-36062: Fix typo in docstring
This corrects a typo in a docstring concerning subgroups of the modular
group. The typo made a definition mathematically incorrect. See issue
#36058
- [x ] The title is concise, informative, and self-explanatory.
- [ x] The description explains in detail what this PR is about.
- [ x] I have linked a relevant issue or discussion.
- [ x] I have updated the documentation accordingly.
### :hourglass: Dependencies
No dependencies: based on 10.1.beta9
URL: https://github.com/sagemath/sage/pull/36062
Reported by: John Cremona
Reviewer(s): David Coudert
