fix: concatenate java extension when DemoSource is defaulted

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -103,7 +103,7 @@ public void addDemo(Component demo) {
       if (sourceCodeUrl.equals(DemoSource.GITHUB_SOURCE)) {
         sourceCodeUrl = Optional.ofNullable(this.getClass().getAnnotation(GithubLink.class))
             .map(githubLink -> githubLink.value() + "/blob/master/src/test/java/"
-                + demo.getClass().getName().replace('.', '/'))
+                + demo.getClass().getName().replace('.', '/') + ".java")
             .orElse(null);
       }
     }