read repo_name when opening repos

src/lib.rs

@@ -69,7 +69,8 @@ pub mod atom;
 /// ```
 /// use gentoo_utils::repo::Repo;
 ///
-/// let repo = Repo::new("/var/db/repos/gentoo");
+/// let repo = Repo::new("/var/db/repos/gentoo")
+///     .expect("failed to open repo");
 ///
 /// for result in repo.categories().expect("failed to read categories") {
 ///    let category = result.expect("failed to read category");