Update urdf_appendix.md

[vidurvij-Unity]

Proposed change(s)

The links in the urdf_appendix.md have been corrected.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

#298

Types of change(s)

• Bug fix
• New feature
• Code refactor
• [x ] Documentation update
• Other (please describe)

Testing and Verification

Clicked the links in GitHub web application and all links are working correctly.

Test Configuration:

• Github on Apple Safari

Checklist

• [ x ] Ensured this PR is up-to-date with the dev branch
• [ x ] Created this PR to target the dev branch
• Followed the style guidelines as described in the Contribution Guidelines
• Added tests that prove my fix is effective or that my feature works
• Updated the Changelog and described changes in the Unreleased section
• Updated the documentation as appropriate

Other comments