[xmonad] Component-Contrib: patch LayoutScreens.hs

Hello haskellers, Configuring my first xmonad start, I ran into a little misspelling mistake in the LayoutScreens.hs usage notes. The patch is, I think, self explanatory. Thank you -- I've finally learned what "upward compatible" means. It means we get to keep all our old mistakes. -- Dennie van Tassel