[HTTP Cache] Validation model: Fix header name

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

In the cache validation model, the response sends an `ETag` header, but the following requests send an `If-None-Match` header. So the `If-None-Match` header from the request is compared to the `ETag` from the previous response. See `Symfony\Component\HttpFoundation\Request::getETags()` which is used by `Symfony\Component\HttpFoundation\Response::isNotModified()`.