Fixed typo on line 20. 

Fully working, annotated version, test script below:

> b <- matrix(1:4, ncol=2)
> a <- makeCacheMatrix(b)
> cacheSolve(a)
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5
> cacheSolve(a)
getting cached data
     [,1] [,2]
[1,]   -2  1.5
[2,]    1 -0.5