latest Primer edits

Hi all,

I've done an edit on the Primer to factor in the latest version of the 
Syntax. I've kept my edits to the bare minimum, and interestingly there 
wasn't much to update: I just completed section 3.4 by adding a chaining 
example with @about, and fixed the use of @src in 3.5 and 3.6, with 
corresponding updates in Section 4 (the triples).

http://www.w3.org/2006/07/SWD/RDFa/primer/

(no frozen version yet, as I'm going to go over the doc for typos and such.)

If you have time, comments are welcome, of course!

-Ben