minor #8874 Replaced class per id in service definition in XML (nacmartin)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #8874).

Discussion
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Replaced class per id in service definition in XML

Using only `class` results in `Top-level services must have "id" attribute, none found`. Whereas using `id` instead of `class` is understood, and it is coherent with the YAML and PHP versions of the same definition.

Commits
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2e0e63e Replaced class per id in service definition in XML

Author: xabbuh

service_container/alias_private.rst

@@ -116,7 +116,7 @@ services.
                 http://symfony.com/schema/dic/services/services-1.0.xsd">
 
             <services>
-                <service class="AppBundle\Mail\PhpMailer" public="false" />
+                <service id="AppBundle\Mail\PhpMailer" public="false" />
 
                 <service id="app.mailer" alias="AppBundle\Mail\PhpMailer" />
             </services>