Update reference from Rustonomicon to Rust Reference

The exercise refers to the "Data layout" section of the Rustonomicon, but now that section claims itself deprecated and redirects to the "Type layout" section of The Rust Reference.

Author: LucaBoschetto

exercises/03_ticket_v1/09_heap/src/lib.rs

@@ -21,8 +21,8 @@ mod tests {
         // This is a tricky question!
         // The "intuitive" answer happens to be the correct answer this time,
         // but, in general, the memory layout of structs is a more complex topic.
-        // If you're curious, check out the "Data layout" section of the Rustonomicon
-        // https://doc.rust-lang.org/nomicon/data.html for more information.
+        // If you're curious, check out the "Type layout" section of The Rust Reference
+        // https://doc.rust-lang.org/reference/type-layout.html for more information.
         assert_eq!(size_of::<Ticket>(), todo!());
     }
 }