docs: update vision image URL example

[HenryLee789]

Summary

• replace the README vision example image URL that now returns an HTTP 400 with a reachable public image URL
• add detail: "low" to keep the example lightweight

Fixes #2776

Testing

• verified the replacement image URL returns HTTP 200 with image/jpeg content
• ran git diff --check

[marcuswood-oai]

Thanks for taking the time to help fix this! We’ve merged #3849 using the image from our vision docs and verified the example works. Closing this as superseded—we appreciate your contribution.