finish until lifetimes

exercises/16_lifetimes/lifetimes3.rs

@@ -1,11 +1,12 @@
 // Lifetimes are also needed when structs hold references.
 
 // TODO: Fix the compiler errors about the struct.
-struct Book {
-    author: &str,
-    title: &str,
+struct Book<'a> {
+    author: &'a str,
+    title: &'a str,
 }
 
+// Will the strings pointed to by book still be valid?
 fn main() {
     let book = Book {
         author: "George Orwell",