Fix broken links in Contributing.md

Contributing.md

@@ -200,8 +200,8 @@ format.
 
 There are different nodes for every kind of item and expression in Rust. For
 more details see the source code in the compiler -
-[ast.rs](https://dxr.mozilla.org/rust/source/src/libsyntax/ast.rs) - and/or the
-[docs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/index.html).
+[ast.rs](https://github.com/rust-lang/rust/blob/master/compiler/rustc_ast/src/ast.rs) - and/or the
+[docs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast/ast/index.html).
 
 Many nodes in the AST (but not all, annoyingly) have a `Span`. A `Span` is a
 range in the source code, it can easily be converted to a snippet of source