Last minute, minor change to navigation menubar example

Team,

 

A comment in issue 410 helped me realize that our navigation menubar example
should have a navigation region around it. Even though it is late in the
review process, I thought the change added sufficient value, is innocuous
enough, and hopefully not contentious enough to cause any problems. Please
comment in the issue if you have any concerns about it or see any errors.

 

I also added an item about it under accessibility features.

 

Navigation menubar example:

http://w3c.github.io/aria-practices/examples/menubar/menubar-1/menubar-1.htm
l

 

Thanks,

Matt