Adding build failed fixes.

Author: the-yash-rajput

src/main/java/com/thealgorithms/graph/TopologicalSort.java

@@ -14,7 +14,7 @@
  * A topological sort is possible only in a directed acyclic graph (DAG).
  * This file contains code of finding topological sort using Depth First Search technique.
  */
-public class TopologicalSort {
+public final class TopologicalSort {
 
     // Prevent instantiation
     private TopologicalSort() {