minor symfony#9222 fix creating a CacheKernel in HTTP cache documentation (danrot)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes symfony#9222).

Discussion
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fix creating a CacheKernel in HTTP cache documentation

Hey, I was trying out to use the HTTP Cache for a talk and lecture I am giving, and with the instructions in the HTTP Cache documentation I only got an error that the class was not found. The changes I introduced here were also necessary. However, I am not completely sure if I missed something else, since I just skimmed the documentation, but I thought I send a PR anyway so that you see what I am talking about 😃

Commits
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cc09426 fix creating a CacheKernel in HTTP cache documentation

Author: javiereguiluz

http_cache.rst

@@ -80,6 +80,8 @@ but is a great way to start.
 To enable the proxy, first create a caching kernel::
 
     // src/CacheKernel.php
+    namespace App;
+
     use Symfony\Bundle\FrameworkBundle\HttpCache\HttpCache;
 
     class CacheKernel extends HttpCache
@@ -93,6 +95,9 @@ caching kernel:
 
     // public/index.php
 
+    use App\Kernel;
+    + use App\CacheKernel;
+
     // ...
     $kernel = new Kernel($_SERVER['APP_ENV'] ?? 'dev', $_SERVER['APP_DEBUG'] ?? ('prod' !== ($_SERVER['APP_ENV'] ?? 'dev')));