Add express Handler import to http-router.md

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

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

@@ -135,6 +135,7 @@ import {
   createRateLimitMiddleware,
 } from '@backstage/backend-defaults/httpRouter';
 import PromiseRouter from 'express-promise-router';
+import { Handler } from 'express';
 import { createServiceFactory } from '@backstage/backend-plugin-api';
 
 const backend = createBackend();