Update SparkJava samples

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

sparkjava/sparkjava/src/main/java/App.java

@@ -4,6 +4,6 @@ public class App {
     public static void main(String[] args) {
         port(8080);
 
-        get("/", (req, res) -> "Hello world");
+        get("/", (req, res) -> "Hello from Docker!");
     }
 }