docs: adding example for singular period

subjects/java/checkpoints/monthly-period/README.md

@@ -21,6 +21,7 @@ public class MonthlyPeriod {
     }
 }
 ```
+
 ### Usage
 
 Here is a possible `ExerciseRunner.java` to test your class:
@@ -30,20 +31,21 @@ public class ExerciseRunner {
     public static void main(String[] args) {
         MonthlyPeriod MonthlyPeriod = new MonthlyPeriod();
 
-        // Test case 1
         String startDate1 = "2020-01-01";
         String endDate1 = "2023-06-15";
         System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate1, endDate1));
 
-        // Test case 2
         String startDate2 = "2015-05-20";
         String endDate2 = "2015-10-20";
         System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate2, endDate2));
 
-        // Test case 3
         String startDate3 = "2018-12-25";
         String endDate3 = "2021-12-25";
         System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate3, endDate3));
+
+        String startDate3 = "2018-10-25";
+        String endDate3 = "2019-11-30";
+        System.out.println("Period: " + MonthlyPeriod.calculatePeriod(startDate3, endDate3));
     }
 }
 ```
@@ -56,5 +58,6 @@ $ java -cp build ExerciseRunner
 Period: 3 years and 5 months
 Period: 5 months
 Period: 3 years
+Period: 1 year and 1 month
 $
 ```