_layouts/webpage.html: specify the document typeof on html, not body

The way Google's structured data extractor handles RDFa does not
associate triples declared before a typeof with the object specified
in that typeof. That is, given the markup:

    <html prefix="...">
    <head>
    <title property="dc:title">Example</title>
    </head>
    <body typeof="s:BlogPost">
    ...
    </body>
    </html>

the dc:title property is not associated with the s:BlogPost, so you
see two rdfa-node entries in the Google Structured Data Testing Tool:

    rdfa-node
    property:
        title:    Example

    rdfa-node
    type:         BlogPost

Moving the typeof declaration to the html node results in the correct:

    rdfa-node
    type:         BlogPost
    property:
        title:    Example

This affects the following tools:
  *  Google Structured Data Testing Tool
  *  the Google search engine
  *  the Structured Data section of the Webmaster Tools