added url to new demonstration video

assignment/socket_chat--patterns-and-testing.md

@@ -106,7 +106,7 @@ authoritative source of requirements.**
 
 Look over the starter code, view
 <!--[this short demonstration](https://use.vg/qB6zFZ),-->
-[this short demonstration](TODO) <!-- this one includes relay server -->
+[this short demonstration](https://use.vg/eiLVWB) <!-- this one includes relay server -->
 and run the program to get a
 feel for how the program works.