add missing closing bracket (vercel#26375)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.

## Documentation / Examples

- [ ] Make sure the linting passes

docs/advanced-features/security-headers.md

@@ -20,6 +20,7 @@ async headers() {
       source: '/(.*)',
       headers: securityHeaders
     }
+  ]
 }
 ```