Fix typo in README.md

[im-renpei]

Update TL;DR section and Python dependencies in README.md

Hi @wbthomason This RP updates the documentation in the following ways:

1. Fix a reference issue in the TL;DR section:

   • Reference [3] has been added to properly cite the MotionBenchMaker dataset.

2. Update Python development headers installation for Ubuntu 22.04:

   • Change from python-dev to python3-dev.

No code changes were made in this commit.

[wbthomason]

Thanks for catching these!