fix: Remove trailing spaces from SudokuSolver.java

- Remove trailing spaces from line 128
- Fix Checkstyle violations

Author: harsha08-2k6

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

@@ -125,6 +125,6 @@ public static void main(String[] args) {
         } else {
             System.out.println("\nNo solution exists for this Sudoku puzzle.");
         }
-        
+
     }
 }