Update ex1_9.cpp

it sums 0-50 rather than 50-100

ch01/ex1_9.cpp

@@ -15,6 +15,6 @@ auto sum(int lo, int hi)
 
 int main()
 {
-    cout << "sum is: " << sum(0, 50 + 1) <<endl;
+    cout << "sum is: " << sum(50, 100+1) <<endl;
     return 0;
 }