Rename simple-chatbot's README.MD to the README.md its pyproject declares

hatchling validates project.readme before building any target, so on a
case-sensitive filesystem `uv sync --all-packages` / `uv build
--all-packages` failed for the whole workspace with "Readme file does not
exist: README.md". The conformance runner's built-in python-sdk config
works around exactly this with two targeted syncs.

No-Verification-Needed: example file rename only

Author: maxisbey