[Bug]: Wrong Api_host definition in VertexAI examples

### File Name

gemini/chat-completions/intro_chat_completions_api.ipynb

### What happened?

The doc incorrectly refers to the location specific api to also having a location specific subdomain. My patch fixes that

Fix:
https://github.com/GoogleCloudPlatform/generative-ai/compare/main...MerzDaniel:generative-ai:patch-1#diff-8e57a7af06dbc565e842aedbbd36d316edf8dadfaee56a3a6ecb93905fe4a709

### Relevant log output

```shell

404. That’s an error.

The requested URL /v1beta1/projects/XXX/locations/eu-west3/endpoints/openapi/chat/completions was not found on this server. That’s all we know.

With correct domain (without location prefix on subdomain) the examples work
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct