Update ex13_31.h
Update ex13_31.h
laohei889/CppPrimer,zhangzhizhongz3/CppPrimer,zhangzhizhongz3/CppPrimer,laohei889/CppPrimer,cheng668/CppPrimer,Sublele/CppPrimer,Sublele/CppPrimer,cheng668/CppPrimer,pezy/CppPrimer,akanezorap/CppPrimer,akanezorap/CppPrimer,pezy/CppPrimer