fix snap() code/example

Now `snap()` syntax is...
```
snap(<length> <length>? [, top | bottom | near ]?) 
```
http://figures.spec.whatwg.org/#floating-to-nearest-edge:-snap
http://dev.w3.org/csswg/css-page-floats/#floating-to-the-topbottom-top-bottom-sna

But, if this syntax is wrong for you as a this spec editor, please close this pull request.