Update the example to reflect current API

[baijum]

What does this PR do?

Update the example to reflect current API

Test Plan

Copy paste the example in the README and check the output. Without this change an error is raised:

python3 hello.py 
Traceback (most recent call last):
  File "/Users/bmuthuka/wa/meta-llama/baiju-experiments/hello.py", line 8, in <module>
    response = client.inference.chat_completion(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/miniconda3/envs/myml/lib/python3.12/site-packages/llama_stack_client/_utils/_utils.py", line 274, in wrapper
    raise TypeError(msg)
TypeError: Missing required arguments; Expected either ('messages' and 'model_id') or ('messages', 'model_id' and 'stream') arguments to be given