fix(structs3.rs): assigned value to cents_per_gram in test

Intended to simplify the lesson by removing the need to figure out what the value is meant to be based on the tests.

Previous commits (9ca08b8f2b and 114b54cbdb (diff-ce1c232ff0ddaff909351bb84cb5bff423b5b9e04f21fd4db7ffe443e598e174)) removed the mathematical complexity, and I feel this addition is a needed change to further streamline the exercise.

exercises/structs/structs3.rs

@@ -73,7 +73,7 @@ mod tests {
         let sender_country = String::from("Spain");
         let recipient_country = String::from("Spain");
 
-        let cents_per_gram = ???;
+        let cents_per_gram = 3;
 
         let package = Package::new(sender_country, recipient_country, 1500);