Tile radio button bug - bottom margin on last item

[maya]

Describe the bug

There's bottom margin on the last radio button of the tile variant. USWDS uses only top margins for consistency. This gives an inconsistent margin below and makes the button margin below it too large. B/c the radio buttons are inside a fieldset element margins will not collapse.

What I see

What I expect

[mejiaj]

Related to #4153

[maya]

@mejiaj this is fixed in #4181. It's pretty straightforward. Can you review it?

[mejiaj]

Sorry, I've been focused on some other work. I've added it to our sprint board so we don't forget.