Re-direct to the right place.

Why redirect Tutorial -> Guide -> Book when you can just Tutorial -> Book?

Suggested here: http://www.reddit.com/r/rust/comments/2schav/is_it_possible_to_automatically_redirect/

src/doc/tutorial.md

@@ -1,3 +1,3 @@
 % The Rust Tutorial
 
-This tutorial has been deprecated in favor of [the Guide](guide.html). Go check that out instead!
+This tutorial has been deprecated in favor of [the Book](book/index.html). Go check that out instead!