Point the Sponsor button at the current GitHub username

The account behind it was renamed from LINUXexpert-org to jcoffey-dev,
and GitHub does not redirect the old name: github.com/sponsors/
LINUXexpert-org answers 404 while the new one answers 200. So the
Sponsor button on this repository has been leading nowhere.

Worth fixing rather than leaving to redirect, because a released
username can be registered by anyone -- a stale link stops being a dead
end and starts being someone else's page.

.github/FUNDING.yml

@@ -1,4 +1,4 @@
 # Funding platforms shown behind the repository's Sponsor button.
 # https://docs.github.com/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository
 
-github: LINUXexpert-org
+github: jcoffey-dev