cleanup: 代码块统一，便于粘贴复制用于测试

src/basic/lifetime.md

@@ -98,9 +98,7 @@ fn main() {
     let result = longest(string1.as_str(), string2);
     println!("The longest string is {}", result);
 }
-```
 
-```rust
 fn longest(x: &str, y: &str) -> &str {
     if x.len() > y.len() {
         x