Fix broken link to Argoverse website in motion forecasting readme

[senselessdev1]

PR Summary

Before linked to
https://www.argoverse.org/data.html
(a dead link). Changed to link to:
https://www.argoverse.org/av2.html

Testing

In order to ensure this PR works as intended, it is:

• unit tested.
• other or not applicable (additional detail/rationale required)

Compliance with Standards

As the author, I certify that this PR conforms to the following standards:

• Code changes conform to PEP8 and docstrings conform to the Google Python style guide.
• A well-written summary explains what was done and why it was done.
• The PR is adequately tested and the testing details and links to external results are included.