Update 16.2.md

Fix typo in 16.2-3

docs/Chap16/16.2.md

@@ -39,7 +39,7 @@ For row $i$ column $j$, we decide whether or not it would be advantageous to inc
 
 > Suppose that in a $0$-$1$ knapsack problem, the order of the items when sorted by increasing weight is the same as their order when sorted by decreasing value. Give an efficient algorithm to find an optimal solution to this variant of the knapsack problem, and argue that your algorithm is correct.
 
-Suppose in an optimal solution we take an item with $v_1$, $w_1$, and drop an item with $v_2$, $w_2$, and $w_1 > w_2$, $v_1 < v_2$, we can substitude $1$ with $2$ and get a better solution. Therefore we should always choose the items with the greatest values.
+Suppose in an optimal solution we take an item with $v_1$, $w_1$, and drop an item with $v_2$, $w_2$, and $w_1 > w_2$, $v_1 < v_2$, we can substitute $1$ with $2$ and get a better solution. Therefore we should always choose the items with the greatest values.
 
 ## 16.2-4