Rewording according to fmoko suggestion.

exercises/primitive_types/primitive_types6.rs

@@ -8,9 +8,9 @@
 #[test]
 fn indexing_tuple() {
     let numbers = (1, 2, 3);
-    /// Replace below ??? by tuple indexing syntax.
+    /// Replace below ??? with the tuple indexing syntax.
     let second = ???;
 
     assert_eq!(2, second
-        "This is not the 2nd number of the tuple numbers")
+        "This is not the 2nd number in the tuple!")
 }