That will bring you to this window
Now go to your Calendar in your Docs and pick "Settings"
Pick the Calendar you want to embed
Go to the blue HTML button
Click on that Link and copy the URL in the address bar
Paste that URL in the box and hit Save.
That is that!! You don't have to update the website anymore to keep your Calendar up to date. Anytime you change the Calendar in your Docs it will change on your website.