Use 4o-mini but get warnings about 4o

```
from tokencost import calculate_prompt_cost, calculate_completion_cost

model = "gpt-4o-mini"
prompt = [{ "role": "user", "content": "Hello world"}]
completion = "How may I assist you today?"

prompt_cost = calculate_prompt_cost(prompt, model)
```

And I got
```
Warning: gpt-4o may update over time. Returning num tokens assuming gpt-4o-2024-05-13
```

But I am using 4o-mini