[whatwg] Discovered typing error in html5

[konstantin.scheumann at viaregia.ch]

Hello!

I think this is a typing error:

Once all the nodes have been processed as described above, the user  
agent must the post-process the menu as follows:

and should mean ...

Once all the nodes have been processed as described above, the user  
agent must then post-process the menu as follows:

kind regards