doc(undergrad.yaml): add equiv.perm.trunc_swap_factors (#7021)
This looks to me like the definition being asked for
```lean
def equiv.perm.trunc_swap_factors {α : Type u} [decidable_eq α] [fintype α] (f : equiv.perm α) :
trunc {l // l.prod = f ∧ ∀ (g : equiv.perm α), g ∈ l → g.is_swap}
```
I suppose the trunc could be considered a problem, but sorting the list is an easy way out, as is `trunc.out` for undergrads who don't care about computability.
Co-authored-by: Floris van Doorn <fpvdoorn@gmail.com>
