Updated tags for OpenAI credentials refresher. (GoogleCloudPlatform#12083)

## Description

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## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.9` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/CODEOWNERS) with the codeowners for this sample
- [ ] This sample adds a new **Product API**, and I updated the [Blunderbuss issue/PR auto-assigner](https://togithub.com/GoogleCloudPlatform/python-docs-samples/blob/main/.github/blunderbuss.yml) with the codeowners for this sample
- [x] Please **merge** this PR for me once it is approved

Author: nathreya-google

generative_ai/chat_completions/chat_completions_credentials_refresher.py

@@ -15,13 +15,13 @@
 # Disable linting on `Any` type annotations (needed for OpenAI kwargs and attributes).
 # flake8: noqa ANN401
 
+# [START generativeaionvertexai_credentials_refresher]
 from typing import Any
 
 import google.auth
 import google.auth.transport.requests
 import openai
 
-
 class OpenAICredentialsRefresher:
     def __init__(self, **kwargs: Any) -> None:
         # Set a dummy key here
@@ -40,6 +40,7 @@ def __getattr__(self, name: str) -> Any:
 
             self.client.api_key = self.creds.token
         return getattr(self.client, name)
+# [END generativeaionvertexai_credentials_refresher]
 
 
 def generate_text(project_id: str, location: str = "us-central1") -> object: