cli/block: Improve example values for target argument — heartwood

This visual representation follows the one that clap is generating for defaults and possible values.

 pub struct Args {
     /// A Repository ID or Node ID to block from seeding or following (respectively)
     ///
     /// Example values:
     /// - z6MkiswaKJ85vafhffCGBu2gdBsYoDAyHVBWRxL3j297fwS9 (Node ID)
     /// - rad:z3Tr6bC7ctEg2EHmLvknUr29mEDLH (Repository ID)
     #[arg(value_name = "RID|NID", verbatim_doc_comment)]
     /// [example values: rad:z3Tr6bC7ctEg2EHmLvknUr29mEDLH, z6MkiswaKJ85vafhffCGBu2gdBsYoDAyHVBWRxL3j297fwS9]
     #[arg(value_name = "RID|NID")]
     pub(super) target: Target,
 }