message: updated yank error message
diff: @@ -801,7 +801,7 @@ pub fn yank(
  };
  let version = match version {
  Some(v) => v,
  - None => bail!("a version must be specified to yank"),
  + None => bail!("a version must be specified to yank (use --vers <VERSION>)"),
  };
  let (mut registry, _) = registry(config, token, index, reg, true, true)?;
  