Update cachematrix.R

Complete the codes of "makeCacheMatrix" and "cacheSolve".
But I am afraid that the first function might have some problem, because I don't really understand the meaning of the symbol "<<-", and it also confused me when I read the example of the function "makeVector", if you are interest in this function, you can see it here:https://github.com/rdpeng/ProgrammingAssignment2