Update XML snippet in "Define Services with a Custom Tag" section

Author: RageZBla

service_container/tags.rst

@@ -215,7 +215,7 @@ For example, you may add the following transports as services:
                     <tag name="app.mail_transport"/>
                 </service>
 
-                <service class="\Swift_SendmailTransport">
+                <service id="Swift_SendmailTransport">
                     <tag name="app.mail_transport"/>
                 </service>
             </services>