fix example in `string-slice.md`

src/basic/compound-type/string-slice.md

@@ -292,8 +292,7 @@ fn main() {
     s.push('r');
     println!("追加字符 push() -> {}", s);
 
-    s.push_str("ust!")
+    s.push_str("ust!");
-        ;
     println!("追加字符串 push_str() -> {}", s);
 }
 ```