Add missing ; to the end of definition of buffer_t (#303)

Co-authored-by: anjiang <anjiang@coremail.cn>

Author: frederick-vs-ja

book/en-us/02-usability.md

@@ -878,7 +878,7 @@ public:
     void free(T& item);
 private:
     T data[BufSize];
-}
+};
 
 buffer_t<int, 100> buf; // 100 as template parameter
 ```

book/zh-cn/02-usability.md

@@ -788,7 +788,7 @@ public:
     void free(T& item);
 private:
     T data[BufSize];
-}
+};
 
 buffer_t<int, 100> buf; // 100 作为模板参数
 ```