refactor: remove code smell java:S5786 (#2159)

https://sonarcloud.io/organizations/iluwatar/rules?open=java%3AS5786&rule_key=java%3AS5786

monad/src/test/java/com/iluwatar/monad/MonadTest.java

@@ -33,7 +33,7 @@ import org.junit.jupiter.api.Test;
 /**
  * Test for Monad Pattern
  */
-public class MonadTest {
+class MonadTest {
 
   @Test
   void testForInvalidName() {