Use Vec.last() method to access the top item of stack. (#942)

* Use Vec.last() method to access the top item of stack.

* Use Vec.last() method to access the top item of stack.

codes/rust/chapter_stack_and_queue/stack.rs

@@ -22,7 +22,7 @@ pub fn main() {
     print_util::print_array(&stack);
 
     // 访问栈顶元素
-    let peek = stack.get(stack.len() - 1).unwrap();
+    let peek = stack.last().unwrap();
     print!("\n栈顶元素 peek = {peek}");
 
     // 元素出栈