fix: add hints to generics1 and generics2 exercises

Author: juanprq

exercises/generics/generics1.rs

@@ -1,6 +1,8 @@
 // This shopping list program isn't compiling!
 // Use your knowledge of generics to fix it.
 
+// Execute `rustlings hint generics1` for hints!
+
 // I AM NOT DONE
 
 fn main() {

exercises/generics/generics2.rs

@@ -1,6 +1,8 @@
 // This powerful wrapper provides the ability to store a positive integer value.
 // Rewrite it using generics so that it supports wrapping ANY type.
 
+// Execute `rustlings hint generics2` for hints!
+
 // I AM NOT DONE
 
 struct Wrapper {