doc: update LICENSE link to main branch

[nschonni]

Fixes #2286

Description

GitHub redirects correctly to the main branch, but this should be updated to skip that step as noted in #2286

Motivation and Context

Testing Details

Example Output(if appropriate)

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[nschonni]

I'm not sure if the link failures are temporary or permanent, but are unrelated to these changes

Sites are back up.
There was a note on the old musl domain:

The new site simplifies and cleans up design, works on mobile browsers, credits sponsors, and brings important information to the main page.

This landing page will be converted to a redirect once all content is moved. The (mostly unmaintained) libc comparison is still available on etalabs.net.

I updated the musl link, but left the other link that they mention might be out-of-date