fix(structs3): Add a hint for panic (#608)

as a totally newbie to Rust, I don't know panic statement from https://doc.rust-lang.org/book/ and rustlings in the beginning. After a hard searching of [should_panic], then I figure out panic statement. 

So it's helpful to tell the learner that write a panic statement here.

exercises/structs/structs3.rs

@@ -16,7 +16,7 @@ struct Package {
 impl Package {
     fn new(sender_country: String, recipient_country: String, weight_in_grams: i32) -> Package {
         if weight_in_grams <= 0 {
-            // Something goes here...
+            // panic statement goes here...
         } else {
             Package {
                 sender_country,