Update ex7_27.cpp

I use  following codes test the  differences of "contents(" ", h * w)" and  " contents(h * w,'  ') "

	std::string a( "a", 10 );
	for (auto i : a)
		std::cout << i;
	std::cout << std::endl;
	std::string b(10,'b' );
	for (auto i : b)
		std::cout << i;