Add missing link for [Node] (rust-lang#2177)

Author: marxin

src/hir.md

@@ -106,6 +106,7 @@ These identifiers can be converted into one another through the [HIR map][map].
 [`LocalDefId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.LocalDefId.html
 [`HirId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir_id/struct.HirId.html
 [`BodyId`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.BodyId.html
+[Node]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.Node.html
 [`CrateNum`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.CrateNum.html
 [`DefIndex`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefIndex.html
 [`Body`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Body.html