[css-ruby-1] Ed nit: missing slash

2.4. Autohiding Base-identical Annotations
https://drafts.csswg.org/css-ruby-1/#autohide

```
<ruby>
  <rb>振</rb><rb>り</rb><rb>仮</rb><rb>名</rb>
  <rp>(</rp><rt>ふ</rt><rt>り</rt><rt>が</rt><rt>な</rt><rp>)</rp>
<ruby>
```

The closing ruby tag needs a /.