[ref] fix build (bad find&replace)

Author: Byron

git-object/tests/immutable/tag.rs

@@ -9,10 +9,7 @@ mod method {
     fn target() -> crate::Result {
         let fixture = fixture_bytes("tag", "signed.txt");
         let tag = Tag::from_bytes(&fixture)?;
-        assert_eq!(
-            tag.target.borrow(),
-            hex_to_id("ffa700b4aca13b80cb6b98a078e7c96804f8e0ec")
-        );
+        assert_eq!(tag.target(), hex_to_id("ffa700b4aca13b80cb6b98a078e7c96804f8e0ec"));
         assert_eq!(tag.target, "ffa700b4aca13b80cb6b98a078e7c96804f8e0ec".as_bytes());
         Ok(())
     }