Merge pull request TheAlgorithms#608 from qckzr/patch-1, fix TheAlgorithms#607

Update README.md

Author: yanglbme

README.md

@@ -58,7 +58,7 @@ 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]