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。
OK, I read the question again, and it ask we use `while` loop.

> Use a while loop to read the text one word at a time. Use the break statement to terminate the loop if a word occurs twice in succession. 

```cpp
while ( cin >> read )
        if (read == tmp && isupper(read[0])) break;
        else tmp = read;
```