minor #18063 [HttpCache] Fix decorating http_cache xml example (HypeMC)

This PR was merged into the 5.4 branch.

Discussion
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[HttpCache] Fix decorating `http_cache` xml example

Followup to #17972 . The xml example is missing the `<services>` tag.

Commits
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6641fa5 [HttpCache] Fix decorating http_cache xml example

Author: javiereguiluz

http_cache/cache_invalidation.rst

@@ -110,11 +110,13 @@ Then, register the class as a service that :doc:`decorates </service_container/s
             xsi:schemaLocation="http://symfony.com/schema/dic/services
                 https://symfony.com/schema/dic/services/services-1.0.xsd"
         >
-            <service id="App\CacheKernel" decorates="http_cache">
-                <argument type="service" id="kernel"/>
-                <argument type="service" id="http_cache.store"/>
-                <argument type="service" id="esi" on-invalid="null"/>
-            </service>
+            <services>
+                <service id="App\CacheKernel" decorates="http_cache">
+                    <argument type="service" id="kernel"/>
+                    <argument type="service" id="http_cache.store"/>
+                    <argument type="service" id="esi" on-invalid="null"/>
+                </service>
+            </services>
         </container>
 
     .. code-block:: php