Update cachematrix.R

Tested with:
m1 <- matrix(c(1,0,5,2,1,6,3,4,0),3,3)
m2 <- matrix(c(1,0,6,2,1,6,3,4,0),3,3)
mcm <- makeCacheMatrix(m1)
> cacheSolve(mcm)
     [,1] [,2] [,3]
[1,]  -24   18    5
[2,]   20  -15   -4
[3,]   -5    4    1
> mcm$set(m2)
> cacheSolve(mcm)
     [,1] [,2]       [,3]
[1,]   -4    3  0.8333333
[2,]    4   -3 -0.6666667
[3,]   -1    1  0.1666667