moved the package in notebooks from pyautogen to autogen

Why are these changes needed?

to help the developers use the autogen package and not the pyautogen, i've only updated the notebooks and not the entire files.

[ ] I've included any doc changes needed for https://autogen-ai.github.io/autogen/. See https://autogen-ai.github.io/autogen/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.