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>

docs/undergrad.yaml

@@ -97,7 +97,7 @@ Group Theory:
     primitive complex roots of unity: 'complex.is_primitive_root_iff'
   Permutation group:
     permutation group of a type: 'equiv.perm'
-    decomposition into transpositions: ''
+    decomposition into transpositions: 'equiv.perm.trunc_swap_factors'
     decomposition into cycles with disjoint support: 'equiv.perm.trunc_cycle_factors'
     signature: 'equiv.perm.sign'
     alternating group: 'alternating_subgroup'