Doc fix: Do not mention next project in book/guessing-game
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
https://github.com/rust-lang/rust/commit/0c6c34de87c899ecb8b977e7ef24510ab2a68168

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

GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust