bug #6451 fix status code in snippet (Barno)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6451).

Discussion
----------

fix status code in snippet

| Q                 | A
| -------------    | ---
| Doc fix?       | yes
| New docs?   | no
| Applies to    | all

Commits
-------

f8f70ba fix status code in snippet

book/http_cache.rst

@@ -920,7 +920,7 @@ Here is how you can configure the Symfony reverse proxy to support the
             if ($this->getStore()->purge($request->getUri())) {
                 $response->setStatusCode(200, 'Purged');
             } else {
-                $response->setStatusCode(200, 'Not found');
+                $response->setStatusCode(404, 'Not found');
             }
 
             return $response;