18-19 Link in Extended Learning for MST needs updated

Oct '18 The link listed given in the Extended Learning section needs to be updated:
from - http://visualgo.net/mst.html
to - https://visualgo.net/en/mst?slide=1

@demetere Thanks for the heads-up. I will send this onto the code.org team. You can send issues like this to support@code.org

The link has been fixed!