Update cachematrix.R

Test performed to validate the function:
X1 <- makeCacheMatrix()
> X1$set(x)
> X1$get()
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> cacheSolve(X1)
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5
> cacheSolve(X1)
getting cached data
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5