Add swig to installation instructions for Box2D

[btjanaka]

Description

As discussed in #662 and #677, it appears that swig must be installed separately before the box2d environments can be installed. This PR updates the documentation to reflect this requirement.

Fixes #662

Type of change

• Bug fix (non-breaking change which fixes an issue)

This is more of a "bug" in the documentation.

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [N/A] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [N/A] My changes generate no new warnings
• [N/A] I have added tests that prove my fix is effective or that my feature works
• [N/A] New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Thanks for the PR