There are some mistakes with the README.markdown and Selection Sampling code improvement #998

Brief Intro

#997
I have created this PR for that issue, there are some mistakes in the README files and also a little improvement for the Selection Sampling example function.

More Details

swap(&a[i], &a[r]) can be written like a.swapAt(i, r)
That would be remove the error called "Overlapping accesses to 'a', but modification requires exclusive access; consider calling MutableCollection.swapAt(::)"