Updated average case for Quicksort

A small edit. Average case for quicksort changed from O(n^2) to O(n log n). Sited:
https://en.wikipedia.org/wiki/Quicksort

Author: antfitch

Diff for: README.md

@@ -55,7 +55,7 @@ From [Wikipedia][quick-wiki]: Quicksort (sometimes called partition-exchange sor
 __Properties__
 * Worst case performance	O(n^2)
 * Best case performance	O(n log n) or O(n) with three-way partition
-* Average case performance	O(n^2)
+* Average case performance	O(n log n)
 
 ###### View the algorithm in [action][quick-toptal]