bug: callback flow diagram shows incorrect callback functions for tools

[waltteri]

In the callback flow diagram (/docs/assets/callback_flow.png) the tool callback function names seem to be incorrect.

According to https://google.github.io/adk-docs/callbacks/ they should be:

• before_tool_callback
• after_tool_callback

Instead, the diagram shows "before_model_callback" and "after_model_callback" for both models and tools. I.e. copy-paste error.

Screenshots

[lavinigam-gcp]

@waltteri That is a very good catch, thank you so much! I just raised the PR #132 to fix this. :) Keep the feedback flowing in, we really appreciate the effort you put in to catch such trivial error.