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

RageZBla · web-flow · commit a06aa707033e · 2019-10-17T16:10:26.000+09:00
diff --git a/service_container/tags.rst b/service_container/tags.rst
@@ -183,7 +183,7 @@ 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>
