Doc fix: Do not mention next project in book/guessing-game

The next project refers to the dining-philosophers problem
https://github.com/rust-lang/rust/blob/27a1834ce522e3ec7fe4726b1661de16ee30c503/src/doc/book/dining-philosophers.md

which was removed in
0c6c34d

so removing reference to that project from book/guessing-game

Author: deepak

src/doc/book/guessing-game.md

@@ -988,8 +988,7 @@ fn main() {
 
 # Complete!
 
-At this point, you have successfully built the Guessing Game! Congratulations!
+This project showed you a lot: `let`, `match`, methods, associated
+functions, using external crates, and more.
 
-This first project showed you a lot: `let`, `match`, methods, associated
-functions, using external crates, and more. Our next project will show off
-even more.
+At this point, you have successfully built the Guessing Game! Congratulations!