Fix mir::Constant link target

Author: marxin

src/mir/index.md

@@ -351,5 +351,5 @@ See the const-eval WG's [docs on promotion](https://github.com/rust-lang/const-e
 [`ProjectionElem::Deref`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.ProjectionElem.html#variant.Deref
 [`Rvalue`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.Rvalue.html
 [`Operand`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.Operand.html
-[`mir::Constant`]: https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/mir/struct.Constant.html
+[`mir::Constant`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.Const.html
 [`ty::Const`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Const.html