Fix minicore.rs link (rust-lang#2122)

Author: marxin

src/tests/minicore.md

@@ -51,4 +51,4 @@ impl Copy for Meow {} // `Copy` here is provided by `minicore`
 fn meow() {}
 ```
 
-[minicore]: https://github.com/rust-lang/rust/tree/master/tests/auxiliary/minicore.rs
+[`minicore`]: https://github.com/rust-lang/rust/tree/master/tests/auxiliary/minicore.rs