getting-involved: some minor fixes

[Staudey]

Description

This fixes some minor issues with the "Getting Involved" doc:

• Matrix header linked to itself (I assume this wasn't intentional)
• It's 501(c)(3), with the two last characters both in parentheses
• Beatrice no longer works on Solus, and "myself" is ambiguous (removed whole list instead and linked to OpenCollective "About" page)
• Accidental extra "the"
• The BuddiesOfBudgie org name should be written in CamelCase and is the org, not the repo
• Replaced "repo" with "repository"

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)

[EbonJaeger]

Any chance you could replace the instances of "repo" with "repository?" Thanks!

[Staudey]

Any chance you could replace the instances of "repo" with "repository?" Thanks!

Good point, done! The page might need another look anyway with such things in mind (and an eye on the style guide).

[davidjharder]

FYI staudey, this repository now has a spellchecker action on it. It will scan any changed files. It also flags certain words. "repo" is one of them