fix random error

app/src/main/java/me/szaki/xkcd/xkcdbrowser/ui/main/MainPresenter.java

@@ -64,7 +64,7 @@ public class MainPresenter extends Presenter<MainScreen> {
         new Thread() {
             @Override
             public void run() {
-                Comic c = getComic(r.nextInt(maxId));
+                Comic c = getComic(r.nextInt(maxId-1)+1);
                 currentId = c.getNum();
                 screen.getImage(c);
                 checkFavorite(c);