Update the service container core symfony services tip

I felt the tip was a little clumsy. It appeared to be missing some
words and referenced the reader as if they were the person writing
the documentation.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

Author: colindecarlo

book/service_container.rst

@@ -848,8 +848,8 @@ the framework.
 
 .. tip::
 
-    Be sure that ``swiftmailer`` entry appears in your application
-    configuration. As you mentioned in :ref:`service-container-extension-configuration`,
+    Be sure that the ``swiftmailer`` entry appears in your application
+    configuration. As was mentioned in :ref:`service-container-extension-configuration`,
     the ``swiftmailer`` key invokes the service extension from the
     ``SwiftmailerBundle``, which registers the ``mailer`` service.