Fix PHP Symfony OpenAPI 3 sample location

Created by: ivan-aksamentov

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
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Dear PHP technical committee, please review this PR @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko @renepardon

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

Fixes #3531 (closed) where 2.0 and 3.0 samples for PHP Symfony overwrite each other. Only 9 characters added: openapi3/, the rest of the changes is the generated OpenAPI 3.0 sample in it's new location.