Update formula.md to include mention of Plex agent change

[bullmoose20]

Description

Update formula.md to include mention of Plex agent change

Issues Fixed or Closed

• Fixes #(issue)

Type of Change

Please delete options that are not relevant.

• 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 not work as expected)
• [x ] Documentation change (non-code changes affecting only the wiki)
• Infrastructure change (changes related to the github repo, build process, or the like)

Checklist

• [ x] My code was submitted to the nightly branch of the repository.