fix(primitive_types6): remove 'unused doc comment' warning

Author: Matthew Smillie

exercises/primitive_types/primitive_types6.rs

@@ -8,7 +8,7 @@
 #[test]
 fn indexing_tuple() {
     let numbers = (1, 2, 3);
-    /// Replace below ??? with the tuple indexing syntax.
+    // Replace below ??? with the tuple indexing syntax.
     let second = ???;
 
     assert_eq!(2, second,