Condition should be checking for false

Closes #3266924

commerce_stock_notifications.module

@@ -28,7 +28,7 @@ function commerce_stock_notifications_form_alter(&$form, &$form_state, $form_id)
       $selected_variation = $product->getDefaultVariation();
     }
     // Check if in stock.
-    if (commerce_stock_notifications_check_stock($selected_variation)) {
+    if (!commerce_stock_notifications_check_stock($selected_variation)) {
       $form['commerce_stock_notifications'] = \Drupal::service('form_builder')->getForm(NotificationForm::class, $selected_variation);
       $form['actions']['submit']['#value'] = t('Out of stock');
       $form['actions']['submit']['#disabled'] = TRUE;