Update the codes of backtracking.

codes/csharp/chapter_backtracking/permutations_i.cs

@@ -25,6 +25,7 @@ public class permutations_i {
                 // 尝试：做出选择，更新状态
                 selected[i] = true;
                 state.Add(choice);
+                // 进行下一轮选择
                 backtrack(state, choices, selected, res);
                 // 回退：撤销选择，恢复到之前的状态
                 selected[i] = false;