Update README.md

Update average case of quicksort: n log n TheAlgorithms#607

Author: qckzr

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]