Update ex5_21.cpp

对于输入 "Hello", "world", "Hello"， 输出结果为"Hello occurs twice in succession."，而"Hello"并不是连续出现。 当处理第二次输入时，由于"world"首字母小写，直接结束这次迭代，tmp的值仍然为"Hello"。 应该在continue之前更新tmp的值。
But I think it has some redundancy, one `for` loop is enough.

```cpp
string read;
    for (string tmp; cin >> read; tmp = read)
        if (read == tmp && isupper(read[0])) break;
```