Fix minicore.rs link

Author: marxin

src/tests/minicore.md

@@ -2,7 +2,7 @@
 
 <!-- date-check Oct 2024 -->
 
-[`tests/auxiliary/minicore.rs`][`minicore`] is a test auxiliary for
+[`minicore`] is a test auxiliary for
 ui/codegen/assembly test suites. It provides `core` stubs for tests that need to
 build for cross-compiled targets but do not need/want to run.
 
@@ -25,6 +25,8 @@ Please note that [`minicore`] is only intended for `core` items, and explicitly
 range of tests.
 </div>
 
+[`minicore`]: https://github.com/rust-lang/rust/blob/master/tests/auxiliary/minicore.rs
+
 ## Example codegen test that uses `minicore`
 
 ```rust,no_run