minor #12498 Update XML snippet in "Define Services with a Custom Tag" section (RageZBla)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #12498).

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

I think the service definition was incorrect (I might be wrong)

Commits
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e0eac57 Update XML snippet in "Define Services with a Custom Tag" section

Author: xabbuh

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>