Consisten placeholder for generics1.rs

This may have been a deliberate choice on the part of the author. As a newbie going through the exercises it seems inconsistent with the previous exercises which use `???` as placeholders.

exercises/generics/generics1.rs

@@ -6,6 +6,6 @@
 // I AM NOT DONE
 
 fn main() {
-    let mut shopping_list: Vec<?> = Vec::new();
+    let mut shopping_list: Vec<???> = Vec::new();
     shopping_list.push("milk");
 }