Link to official version of internal API documentation in contributing.md

[phansch]

The CONTRIBUTING.md currently links to http://manishearth.github.io/rust-internals-docs in two places.

Once rust-lang/rust-central-station#40 is merged, there will be a stable location for the internal API documentation and we should link to that instead.

The new API docs will be at https://doc.rust-lang.org/nightly-rustc

[phansch]

The new docs are now live at https://doc.rust-lang.org/nightly/nightly-rustc/rustc/. With c170e86 there is only one link left that will have to be fixed 👍

[Manishearth]

1c73db1