Simplify CC-0 license as a single metatag.

Suggested simplification by @jar398. This metatag converts to the following NeXML:
```
<meta 
   rel="xhtml:license" 
   href="http://creativecommons.org/publicdomain/zero/1.0/" 
   xsi:type="nex:ResourceMeta"
/>
```
(I gather the ```xhtml``` namespace used here is sometimes styled as ```xhv```.)