Update stream chat completions API cookbook

[brianz-openai]

Summary

Motivation

Why are these changes necessary? How do they improve the cookbook?

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[owencm]

lgtm

[owencm]

Actually one quick thought, can we link to the relevant part of the API reference (the usage key in the chunk section and the stream_option section of the request section?)

The current plan is for the tweet to link to this section of this cookbook, so this will get quite a lot of traffic and it'd be nice if it linked through to the other resources nicely.

[owencm]

Also let's mention in this cookbook that choices will be [] in this case, since I think this will require a code change for a lot of people

[brianz-openai]

updated. Also updated the screenshot in the PR description.