docs: add link for the next project in the guessing-game doc

[deepak]

I am learning rust and reading the book for the first time

The guessing-game section mentions a "Next Project" at the very end

Our next project will show off even more.

We can add a link to that project ?

[GuillaumeGomez]

It's referring to the next project showed in the book (so next section if I remember well). If @steveklabnik confirms, this issue can be closed.

[steveklabnik]

Actually, this sentence is vestigial from when we had the Dining Philosophers chapter. I would actually prefer to remove this sentence. Want to submit another PR? :D

[deepak]

@steveklabnik absolutely :-) on it

[deepak]

was interested in looking at the old example.
pasted here for future people
was removed in 0c6c34d

[deepak]

@steveklabnik sent PR