Add return statements to the Fibonacci exercise. (#2576)

At this point in the course, we have not explained return statements.
Better to have it set up to avoid questions!

src/types-and-values/exercise.md

@@ -14,10 +14,10 @@ this function panic?
 {{#include exercise.rs:fib}}
     if n < 2 {
         // The base case.
-        todo!("Implement this")
+        return todo!("Implement this");
     } else {
         // The recursive case.
-        todo!("Implement this")
+        return todo!("Implement this");
     }
 }