Update README.markdown

when k > n/2 ,  we just need random select a.count - k elements and remove them from the original array

Selection Sampling/README.markdown

@@ -193,7 +193,7 @@ print(output.count)
 
 The performance of this second algorithm is **O(n)** as it may require a pass through the entire input array.
 
-> **Note:** If `k > n/2`, then it's more efficient to do it the other way around and choose `k` items to remove.
+> **Note:** If `k > n/2`, then it's more efficient to do it the other way around and choose `a.count - k` items to remove.
 
 Based on code from Algorithm Alley, Dr. Dobb's Magazine, October 1993.