docs(examples): print event.delta in the streaming helpers doc (#3876)

## Summary

The streaming snippet in `helpers.md:172` prints `event.content` for
`content.delta` events, but `ContentDeltaEvent` only has `delta` and
`snapshot` fields (`src/openai/lib/streaming/chat/_events.py`) — running
the snippet raises `AttributeError`. Changed to `event.delta`, which the
same page already documents at line 199.

## Checklist

- [x] I understand that this repository is auto-generated and my pull
request may not be merged

Co-authored-by: Marcus Wood <marcuswood@openai.com>