Feedback for “Completion & Generation”

Api of completion_model has been changed in the latest version. There is no `model.prompt` now.

https://docs.rig.rs/docs/concepts/completion#basic-completion

## Basic Completion
```
let openai = openai::Client::from_env();
let model = openai.completion_model("gpt-4o");

let response = model
.prompt("Explain quantum computing")
.await?;
```

The issue points to outdated documentation for the completion_model API in the Rig Framework. Look at the linked documentation page (docs/concepts/completion) and find the example code block. Compare with the current Rust API to see the correct method names. Update the example to use the current API, likely replacing .prompt() with the new method. Verify by checking the source code or recent examples in the repository.

索引モデルが issue の本文から書いたものです。