[xmonad] Can't use any of the contrib modules

[wagnerdm at seas.upenn.edu]

Quoting Anthony Campbell <ac at acampbell.org.uk>:

> This comes from Xmonad-Doc-Extending, which I found at
> http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Doc-Extending.html#The_LayoutClass

I've pushed a documentation patch fixing this and a few other errors  
in the documentation there.

~d