docs: update generate stream example

[babu-ch]

Description here...

update example

https://firebase.google.com/docs/genkit/models

Checklist (if applicable):

• [ ] Tested (manually, unit tested, etc.)
• [ ] Changelog updated
• [x] Docs updated

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[MichaelDoyle]

Thanks for your contribution! This page was rewritten and this PR no longer applies.

BTW, if you get set up with the CLA, we'll be able to get this merged in quickly next time.