Fix GuildChannel::create_permission examples
Fix GuildChannel::create_permission examples

The examples didn't bring `model::PermissionOverwriteType` into scope.
Additionally, wrap the examples in try_main's to make use of the `?`
operator.

zeyla/serenity.rs,eLunate/serenity,zeyla/serenity,Roughsketch/serenity,acdenisSK/serenity,Flat/serenity,Lakelezz/serenity