Update sort-an-array.md

Author: neetcode-gh

articles/sort-an-array.md

@@ -261,7 +261,7 @@ public class Solution {
 
 ### Time & Space Complexity
 
-* Time complexity: $O(n)$ in average case, $O(n ^ 2)$ in worst case.
+* Time complexity: $O(n \log n)$ in average case, $O(n ^ 2)$ in worst case.
 * Space complexity: $O(\log n)$ for recursive stack.
 
 ---
@@ -1369,4 +1369,4 @@ public class Solution {
 ### Time & Space Complexity
 
 * Time complexity: $O(n \log n)$ in average case, $O(n ^ 2)$ in worst case.
-* Space complexity: $O(1)$
+* Space complexity: $O(1)$