[Docs] Update diagram on Human in the Loop Example

[allisonwhilden]

Why are these changes needed?

Updating documentation assets to match our brand.

Related issue number

Part of #1707

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.