minor symfony#13915 Update injection_types.rst (zairigimad)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Update injection_types.rst

missing annotation

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Commits
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6bfe7e3 Update injection_types.rst

Author: javiereguiluz

service_container/injection_types.rst

@@ -222,6 +222,9 @@ that accepts the dependency::
     {
         private $mailer;
 
+        /**
+         * @required
+         */
         public function setMailer(MailerInterface $mailer)
         {
             $this->mailer = $mailer;