Fix checkstyle " Class Subsequence should be declared as final."

Author: alxkm

src/main/java/com/thealgorithms/backtracking/Subsequence.java

@@ -6,7 +6,7 @@
 /**
  * Class generates all subsequences for a given list of elements using backtracking
  */
-public class Subsequence {
+public final class Subsequence {
     private Subsequence() {
     }