Change private flag description

To highlight the fact that it configures the repository as defined,
which was not _really_ clear to some users and has resulted in public
repositories accidentally being changed to private (losing important
goodies like stars and linked forks).

Discussion on this is ongoing and there will likely be other
improvements in the near future to protect users against this.

Signed-off-by: Hidde Beydals <hello@hidde.co>