Programmatically define `assistant_id` instead of hard coding its value in `run_assistant` function

[MrJarnould]

The code in assistants_quickstart.py will fail to run unless you programmatically define the assistant_id.

i.e. line 94 goes from this: assistant = client.beta.assistants.retrieve("asst_7Wx2nQwoPWSf710jrdWTDlfE")

to this: assistant = client.beta.assistants.retrieve(assistant.id)