Generate baseUrl from correct entrypoint route

Closes #3515617

src/Controller/ExplorerController.php

@@ -45,8 +45,7 @@ class ExplorerController extends ControllerBase {
    *   Render array for the explorer application.
    */
   public function explorer() {
-    $base_url = Url::fromRoute('<front>', [], ['absolute' => TRUE])->toString();
-    $jsonapi_url = $base_url . 'jsonapi';
+    $jsonapi_url = Url::fromRoute('jsonapi.resource_list', [], ['absolute' => TRUE])->toString();
 
     return [
       '#theme' => 'jsonapi_query_builder',