minor #7478 Fix Typo $response->setEtag(..) (gmorel)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7478).

Discussion
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Fix Typo $response->setEtag(..)

instead of $response->setETag(..)

Keep up the good work !

Commits
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9ce1a77 Fix Typo $response->setEtag(..)

Author: xabbuh

http_cache/validation.rst

@@ -69,7 +69,7 @@ To see a simple implementation, generate the ETag as the md5 of the content::
         public function homepageAction(Request $request)
         {
             $response = $this->render('static/homepage.html.twig');
-            $response->setETag(md5($response->getContent()));
+            $response->setEtag(md5($response->getContent()));
             $response->setPublic(); // make sure the response is public/cacheable
             $response->isNotModified($request);