minor #7010 Update service_decoration.rst (lamari)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #7010).

Discussion
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Update service_decoration.rst

add : after class in configuration

Commits
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b7f8257 Update service_decoration.rst

Author: xabbuh

service_container/service_decoration.rst

@@ -18,7 +18,7 @@ the original service is lost:
             # this replaces the old app.mailer definition with the new one, the
             # old definition is lost
             app.mailer:
-                class AppBundle\DecoratingMailer
+                class: AppBundle\DecoratingMailer
 
     .. code-block:: xml