refactor: change groupNumber type from int to String in StudentGroupComparator

app/src/test/java/sorting/StudentGroupComparator.java

@@ -5,7 +5,7 @@ import java.util.Comparator;
 public class StudentGroupComparator implements Comparator<Student> {
 
     @Override
-    public int compare(Student s1, Student s2) {
+    public String compare(Student s1, Student s2) {
         return Integer.compare(s1.getGroupNumber(), s2.getGroupNumber());
     }
 }