First radio button should be `checked` in Vue example

Problem:

We have it as a requirement that one radio button in a set should be selected by default, however, the Vue example does not have one with the checked attribute.

Solution:

Add checked to the first radio button in the group.

Reason:

It’s a good practice to have elements in states where they can be undone. In the case of radio buttons, once one in a group is selected, you can never unselect all to get back to the original state. For this reason, one radio button in a group should have the checked attribute, and typically it’s the first one unless the state reflects a previous choice.

/cc @tauriedavis