minor #10767 Change wrong class name (alexchuin)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #10767).

Discussion
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Change wrong class name

Hi,

I've fixed a little class name error/typo.

Alex

Commits
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4e4eecc Change wrong class name

Author: javiereguiluz

service_container/tags.rst

@@ -265,7 +265,7 @@ container for any services with the ``app.mail_transport`` tag::
             $taggedServices = $container->findTaggedServiceIds('app.mail_transport');
 
             foreach ($taggedServices as $id => $tags) {
-                // add the transport service to the ChainTransport service
+                // add the transport service to the TransportChain service
                 $definition->addMethodCall('addTransport', array(new Reference($id)));
             }
         }