modify the python filename extraction

[JieyuZ2]

Why are these changes needed?

modify from code[11 : code.find("\n")].strip() to code[11 : code.find(".py")].strip() + '.py'

otherwise, invalid python file name would cause issue in committing current docker

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#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.

[sonichi]

Please use https://microsoft.github.io/FLAML/docs/Contribute#pre-commit to pass code format check.