gh-35215: Trac #34920: Fix description of inputs of primes.
Fix docstring of the `primes` function.
### 📚 Description
The documentation of `primes` states that "stop" is optional and "start"
is required, when in fact it is the opposite.
Closes #34920
### 📝 Checklist
- [x] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [x] I have linked an issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### ⌛ Dependencies
URL: https://github.com/sagemath/sage/pull/35215
Reported by: Eloi Torrents
Reviewer(s): Eloi Torrents, trevorkarn
