Update ex5_14.cpp
Update ex5_14.cpp

When I look at ch05, I don't konw "pair"!
laohei889/CppPrimer,zhangzhizhongz3/CppPrimer,Sublele/CppPrimer,pezy/CppPrimer,cheng668/CppPrimer,cheng668/CppPrimer,akanezorap/CppPrimer,laohei889/CppPrimer,zhangzhizhongz3/CppPrimer,Sublele/CppPrimer,pezy/CppPrimer,akanezorap/CppPrimer