Arreglat error en el mètode getClasses

Change-Id: If441ff22736b8dfbcd69a9e98d04005b46d91c90
diff --git a/inc/Classes.php b/inc/Classes.php
index 5059040..9c21ac8 100644
--- a/inc/Classes.php
+++ b/inc/Classes.php
@@ -29,7 +29,7 @@
     $classes = $query->fetchAll(\PDO::FETCH_ASSOC);
 
     foreach ($classes as &$class) {
-      if ($isSignedIn)
+      if (!$isSignedIn)
         $class['user_subject_id'] = null;
 
       $class['user_selected'] = $class['user_subject_id'] !== null;