docs: Change saved/configured chain variable name (#18863)
**Description:**
Variable name was `openai_poem` but it didn't pass in the `"prompt":
"poem"` config, so the examples were showing a joke being returned from
a variable called `*_poem`.
We could have gone one of two ways:
1. Updating the config line and the output line, or
2. Updating the variable name
The latter seemed simpler, so that's what I went with. But I'd be glad
to re-do this PR if you prefer the former.
Thanks for everything, y'all. You rock 🤘
**Issue:** N/A
**Dependencies:** N/A
**Twitter handle:** `conroywhitney`
