fix: correct permutation iterator offset for k index

Adjust iterator advancement to account for 1-based k indexing.
This fixes returning the wrong permutation when selecting the k-th
result.

3518/solution.cpp

@@ -34,7 +34,7 @@ public:
     }
 
     auto it = permutations.begin();
-    advance(it, k);
+    advance(it, k - 1);
 
     string mirroredHalf = *it;
     reverse(mirroredHalf);