remove dependency on 'pickle5' module

[seung0h]

While I'm doing unit 2, I faced a tiny issue on installing dependencies.

Because 'pickle5' has been deprecated after Python 3.8 (https://pypi.org/project/pickle5/), I removed its dependency from the notebook.

This simple contribution would be helpful to new learners.

Thank you!

[mriusero]

I confirm that using 'import pickle' instead of 'pickle5' works correctly for Unit 2, both for uploading and downloading.