Update backtracking/subset_sum.cpp

Co-authored-by: Filip Hlasek <fhlasek@gmail.com>

Author: Swastyy

backtracking/subset_sum.cpp

@@ -18,7 +18,7 @@ namespace backtracking {
 /**
  * @namespace Subsets
  * @brief Functions for counting subsets in a given array with a given sum
- * Time Complexity: O(sum*n), where the sum is the ‘target sum’ and ‘n’ is the
+ * Time Complexity: O(n * 2^n), where ‘n’ is the number of elements in the given array.
  * size of the array.
  */
 namespace Subsets {