change link to point to Doxidize, the "new new rustdoc"

steveklabnik/rustdoc is currently an "archived", read-only repository;
steveklabnik/rustdoc@04da5c295 instructs the reader to see
steveklabnik/doxidize as the successor project.

Author: zackmdavis

src/rustdoc.md

@@ -4,7 +4,7 @@ Rustdoc actually uses the rustc internals directly. It lives in-tree with the
 compiler and standard library. This chapter is about how it works. (A new
 implementation is also [under way], though).
 
-[under way]: https://github.com/steveklabnik/rustdoc
+[under way]: https://github.com/steveklabnik/doxidize
 
 Rustdoc is implemented entirely within the crate [`librustdoc`][rd]. It runs
 the compiler up to the point where we have an internal representation of a