Add PromiseRouter import to http-router.md

Signed-off-by: Gustavo RPS <516827+gustavorps@users.noreply.github.com>

docs/backend-system/core-services/http-router.md

@@ -134,6 +134,7 @@ import {
   createAuthIntegrationRouter,
   createRateLimitMiddleware,
 } from '@backstage/backend-defaults/httpRouter';
+import PromiseRouter from 'express-promise-router';
 import { createServiceFactory } from '@backstage/backend-plugin-api';
 
 const backend = createBackend();