Fix other broken link

src/librustc/infer/region_constraints/README.md

@@ -1,3 +1,3 @@
 For info on how the current borrowck works, see the [rustc guide].
 
-[rustc guide]: https://rust-lang.github.io/rustc-guide/mir/borrowck.html
+[rustc guide]: https://rust-lang.github.io/rustc-guide/borrow_check.html