Stray reference to fieldset in vertical-tabs.js that was not updated to details

In the port from fieldsets to details elements, one reference to fieldset was not updated to details.

STR.

1. Create a custom block
2. Set the Chrome developer tools to stop on any exception under the Sources tab
3. Try to place the custom block
4. Your browser should get caught on an exception in vertical-tabs.js

Here's a screenshot of the Developer Tools stopped on the exception where settings.fieldset is undefined.

And call in the stack right before that that passes an object with the key details instead of fieldset.

Changing settings.fieldset to settings.details should fix this.