Added useful link about immutable and unmodifiable-collections.

Author: Alexis-Dia

src/by/bntu/fitr/povt/immutable/case4/Test.java

@@ -7,6 +7,7 @@
 
 /**
  * @author Alexey Druzik on 04.11.2020
+ * https://habr.com/ru/company/piter/blog/470149/ (orig - https://nipafx.dev/immutable-collections-in-java#)
  */
 public class Test {
     public static void main(String[] args) {

src/by/bntu/fitr/povt/immutable/case5/Main.java

@@ -11,6 +11,7 @@
 
 /**
  * @author Alexey Druzik on 10.11.2020
+ * https://habr.com/ru/company/piter/blog/470149/ (orig - https://nipafx.dev/immutable-collections-in-java#)
  * Difference between  Immutable collection and Unmodifiable collection:
  *     Unmodifiable collection(also calls Structually Modified) - Collections that do not support modification operations
  *         (such as add, remove and clear) are referred to as unmodifiable. Collections that are not unmodifiable are modifiable.