book: fix incorrect return value description of std::weak_ptr expired (#144)

* Fix error

资源尚存在时，`std::weak_ptr` 的 `expired()` 方法返回 `false`, 否则返回 `true`。

* book: fix error

Fix error, while the resource of `std::shared_ptr` exists, `std::weak_ptr`'s `expired()` method returns `false`, otherwise it returns `true`.

book/en-us/05-pointers.md

@@ -187,7 +187,7 @@ The solution to this problem is to use the weak reference pointer `std::weak_ptr
 
 In the above figure, only B is left in the last step, and B does not have any smart pointers to reference it, so this memory resource will also be released.
 
-`std::weak_ptr` has no `*` operator and `->` operator, so it can't operate on resources. Its only function is to check if `std::shared_ptr` exists, its `expired() The ` method can return `true` when the resource is not released, otherwise it returns `false`.
+`std::weak_ptr` has no `*` operator and `->` operator, so it can't operate on resources. Its only function is to check if `std::shared_ptr` exists, its `expired()` method can return `false` when the resource is not released, otherwise it returns `true`.
 
 ## Conclusion
 

book/zh-cn/05-pointers.md

@@ -175,7 +175,7 @@ int main() {
 
 在上图中，最后一步只剩下 B，而 B 并没有任何智能指针引用它，因此这块内存资源也会被释放。
 
-`std::weak_ptr` 没有 `*` 运算符和 `->` 运算符，所以不能够对资源进行操作，它的唯一作用就是用于检查 `std::shared_ptr` 是否存在，其 `expired()` 方法能在资源未被释放时，会返回 `true`，否则返回 `false`。
+`std::weak_ptr` 没有 `*` 运算符和 `->` 运算符，所以不能够对资源进行操作，它的唯一作用就是用于检查 `std::shared_ptr` 是否存在，其 `expired()` 方法能在资源未被释放时，会返回 `false`，否则返回 `true`。
 
 ## 总结