docs: change the methode name to be alinged with the tests for the singleton-bleurprint

Author: zanninso

subjects/java/checkpoints/singleton-blueprint/ExerciseRunner.java

@@ -1,5 +1,5 @@
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(Singleton.get???().whoAreYou());
+        System.out.println(Singleton.get???().ShowMessage());
     }
 }

subjects/java/checkpoints/singleton-blueprint/README.md

@@ -16,7 +16,7 @@ public class Singleton {
     public Singleton get???() {
     }
 
-    public String whoAreYou() {
+    public String ShowMessage() {
         return "Hello, I am a singleton!"
     }
 }
@@ -29,7 +29,7 @@ Here is a possible `ExerciseRunner.java` to test your class:
 ```java
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(Singleton.get???().whoAreYou());
+        System.out.println(Singleton.get???().ShowMessage());
     }
 }
 ```