docs: Rename API middlewares title in sidebar (#39534)

Based on the PR #39407, the sidebar title for API middlewares is changed from "API Middlewares" to "Request Helpers".




## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm lint`
- [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

docs/manifest.json

@@ -143,7 +143,7 @@
               "path": "/docs/api-routes/dynamic-api-routes.md"
             },
             {
-              "title": "API Middlewares",
+              "title": "Request Helpers",
               "path": "/docs/api-routes/request-helpers.md"
             },
             {