rename helloworld exercise to good-by-mars

subjects/java/checkpoints/good-by-mars/README.md

@@ -1,14 +1,14 @@
-## HelloWorld
+## GoodbyMars
 
 ### Instructions
 
-In a file named `HelloWorld.java` write a function `helloWorld` that returns the string 'Hello World !'.
+In a file named `GoodbyMars.java` write a function `goodbyMars` that returns the string 'Good by Mars !'.
 
 ### Expected Functions
 
 ```java
-public class HelloWorld {
-    public static String helloWorld() {
+public class GoodbyMars {
+    public static String goodbyMars() {
         // your code here
     }
 }
@@ -21,7 +21,7 @@ Here is a possible ExerciseRunner.java to test your function :
 ```java
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(HelloWorld.helloWorld());
+        System.out.println(GoodbyMars.goodbyMars());
     }
 }
 ```
@@ -31,6 +31,6 @@ and its output :
 ```shell
 $ javac *.java -d build
 $ java -cp build ExerciseRunner
-Hello World !
+Good by Mars !
 $
 ```