Fix description of create webhook response

[victorges]

There is no stream object there. It's only a created webhook and the information about it.

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
livepeer-studio-docs	✅ Ready (Inspect)	Visit Preview	Oct 4, 2022 at 4:39PM (UTC)