Update Exercises Directory Names to Reflect Order

exercises/16_lifetimes/lifetimes3.rs

@@ -0,0 +1,21 @@
+// lifetimes3.rs
+//
+// Lifetimes are also needed when structs hold references.
+//
+// Execute `rustlings hint lifetimes3` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+struct Book {
+    author: &str,
+    title: &str,
+}
+
+fn main() {
+    let name = String::from("Jill Smith");
+    let title = String::from("Fish Flying");
+    let book = Book { author: &name, title: &title };
+
+    println!("{} by {}", book.title, book.author);
+}