Update structs.md (#232)

Extending a struct example that demonstrates how to change values and how to partially copy structs.

src/structs.md

@@ -9,11 +9,19 @@ struct Person {
 }
 
 fn main() {
-    let peter = Person {
+    let mut peter = Person {
         name: String::from("Peter"),
         age: 27,
     };
-
     println!("{} is {} years old", peter.name, peter.age);
+    
+    peter.age = 28;
+    println!("{} is {} years old", peter.name, peter.age);
+    
+    let jackie = Person {
+        name: String::from("Jackie"),
+        ..peter
+    };
+    println!("{} is {} years old", jackie.name, jackie.age);
 }
 ```