Fix small bug in doc (#340)

## What this does

<!-- Clear description of what this PR does and why -->
Fix bug in documentation that tripped me up for a few minutes today.

## Type of change

- [x] Bug fix
- [x] Documentation

## Scope check

- [x] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [x] This aligns with RubyLLM's focus on **LLM communication**
- [x] This isn't application-specific logic that belongs in user code
- [x] This benefits most users, not just my specific use case

## Quality check

- [x] I updated documentation if needed
- [x] I didn't modify auto-generated files manually (`models.json`,
`aliases.json`)

## API changes

- [x] No API changes

## Related issues

<!-- Link issues: "Fixes #123" or "Related to #123" -->

Author: tpaulshippy

docs/_advanced/models.md

@@ -260,7 +260,7 @@ puts response.content
 
 # You can also use it in .with_model
 chat.with_model(
-  model: 'gpt-5-alpha',
+  'gpt-5-alpha',
   provider: :openai,                # MUST specify provider
   assume_exists: true
 )