Update ex3_8.cpp
Update ex3_8.cpp

Is it better to define the type of i to decltype(str.size()) rather than int？
andersy005/Cpp-Primer,Mooophy/Cpp-Primer,AndychenCL/Cpp-Primer,Soyn/Cpp-Primer,coldmanck/Cpp-Primer,sanerror/Cpp-Primer,Squawk09/Cpp-Primer,Mooophy/Cpp-Primer,sanerror/Cpp-Primer,Squawk09/Cpp-Primer,Soyn/Cpp-Primer,coldmanck/Cpp-Primer,andersy005/Cpp-Primer,AndychenCL/Cpp-Primer