Remove broken image url

[KevinYang21]

Remove url pointing to a 404 url

Changes summary

Fixes #ISSUE_NUMBER https://github.com/ray-project/ray-educational-materials/issues/111

Summary of changes in this PR Remove a broken image url in the list.

The broken image url can lead to errors in the materials. E.g. Running this cell will get:

UnidentifiedImageError: cannot identify image file '**/ray-educational-materials/Ray_Core/task_images/stennis.jpg'

(image count referred Let's download 100 large images is not updated, as the # of images is 101 even after the removal)

PR scope

• [ ] new notebook
• [ ] improvements to the existing notebook
• [x] bug fixes
• [ ] other

Checklist

• [ ] notebook is without outputs
• [ ] empty cells are removed
• [ ] code is formatted with black
• [ ] imports are sorted
• [ ] if new notebook is added, README is updated