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
Merge pull request #2467 from colindecarlo/service-container-update-core-symfony-services-tip

Update the service container core symfony services tip