Update three_int_sum.md

Author: nataliepjlin

three_int_sum.md

@@ -1,3 +1,4 @@
+https://www.youtube.com/watch?v=lkW5PlJnEo4&list=PLY_qIufNHc293YnIjVeEwNDuqGo8y2Emx&index=17
 ```C
 #include <stdio.h>
 int main(){
@@ -14,4 +15,4 @@ int main(){
     printf("The sum of these three integers is %d.",n1+n2+n3);
     return 0;
 }
-```
+```