Clarify that each philosopher should think/eat 100 times (#900)

Clarify that each philosopher should think/eat 100 times.

Folks who have hazy memories of the philosopher dining problem may
interpret the original instructions as "make each philosopher think and
eat once". This interpretation loses a critical detail, because the
resulting code is highly unlikely to deadlock in practice, even without
breaking the symmetry.

src/exercises/concurrency/dining-philosophers.md

@@ -32,7 +32,7 @@ blanks, and test that `cargo run` does not deadlock:
 
     // Create philosophers
 
-    // Make them think and eat
+    // Make each of them think and eat 100 times
 
     // Output their thoughts
 }