minor #456 Add OskarStark as codeowner for demo project (OskarStark)

This PR was merged into the main branch.

Discussion
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 Add `OskarStark` as codeowner for demo project

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Docs?         | no
| Issues        | --
| License       | MIT

Commits
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f975702 Add `OskarStark` as codeowner for demo project

Author: OskarStark

.github/CODEOWNERS

@@ -1,4 +1,4 @@
-demo @chr-hertel
+demo @chr-hertel @OskarStark
 examples @chr-hertel @OskarStark
 fixtures @chr-hertel @OskarStark
 src @chr-hertel @Nyholm @OskarStark