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)

[Nagiseishiro773]

Cool