Modeling spec: add attributedTo example for 'Repository'

https://notabug.org/peers/forgefed/pulls/100/files

On irc.libera.chat#peers:

(09:26:48) dachary: zPlus: I think you’ll find this pull request to be really trivial then https://notabug.org/peers/forgefed/pulls/100/files :slight_smile:
(09:27:23) dachary: I tried to mimic the structure of the commit title from other commit messages

(09:37:01) dachary: zPlus: thanks for the merge \o/
(09:37:13) zPlus: yw